Those are both examples of expenses, but they kick in at vastly different levels of seriousness.
Let's say that a player decided to take their first step into high-end content with Storm's Crown EX (6.2) at launch. Storm's Crown EX has an official minimum item level of 600, and players take that seriously because the heal checks are designed around it. Going in with less gear than that is putting undue burden on your healers, and PFs will rightly lock it out en masse. At launch, a player who hadn't done high-end content before 6.2 had a maximum item level of... 599, brought down by the one unaugmented ring. So our dabbler has to spend either gil or time getting a current-tier piece to reach minimum item level. But you can do that in Normal Raids or with the new tomes, so that's not so hard, is it?
Being minimum item level or just above, now they're expected to fill the materia slots on all their gear for the first time - with current materia X. Suddenly, their damage output matters. Assuming they got a Normal Raid or crafted ring, with a single materia slot, they have 21 slots to fill: 10 two-slot pieces from the previous tier, and the one one-slot piece they just got. (Other scenarios are worse for their total expenses, requiring more investment in gear or melds.) Using the Aether markets since I see that's where you are, materia X in the desirable substats was often running over 25k each. That means over 500k in materia just to set foot in the fight - or any high-end content while it's current - and it's not even the expansion launch rush on materia! Not for overmelding, but for slot melding.
The fact that they're expected to use raid food for the first time pales in comparison. They can use last tier's (cheaper) food, and EX fights only take a few hours to learn and a few more to get a weapon from, with no weekly commitment. 20k or so of old food will do.
The next week, our dabbler takes their shiny new EX weapon into P5S. (Along with at least one tome piece, possibly two, and more Normal Raid gear, which means 75k or more in materia to refill slots as they upgrade despite taking the materia out of the old pieces.) Assuming they find a party accepting item level 600 instead of going "but everyone had time to get crafted" and demanding 610 or even 612, now they need current raid food. Going in without it is asking to die to raidwides. If they take 3 hours to learn the fight, riding the first wave when PF is good, they've spent 30k in food... and maybe hit enrage instead of getting the clear.
The more serious and experienced raiders, who have more polished rotations and have learned to learn faster, were mostly in the 605 / 610 / 612 parties or in statics. Our dabbler's group, determined to clear despite potentially taking a few deaths or Damage Downs near minimum item level, faces an expectation of Savage but not EX: raid potions.
Raid food is used on a per-unit-time basis. Raid potions are used on a per-pull basis when trying to clear. And P5S is a three-potion fight. (So are P6S and P7S. P8S is four-potion, counting both halves together.) Pushing for that clear on the first day of Savage, while their potions could still be 10k each, they could easily drink down 60 potions trying for that first kill. 600k in potions is no joke, an expense almost as high as everything else they needed to get this far!
And for their trouble, they got... access to [Duty Complete] P5S parties next week, a book, a less than 3/8 chance at getting an accessory (one person can win multiple of the 3 drops), and access to P6S once they hit item level 605. Because Savage, unlike EX, isn't farmable. They can enter P6S without buying crafted gear - their second week of tomestone and Normal Raid gear can get them to 605 alongside that EX weapon - but if they want to guarantee getting gear from P5S, they have to come back week after week. Which means two things:
They have to use food and potions every week. Even after they're well past minimum item level, they'll be expected to use food for heal planning and incentivized to use potions to maybe salvage close pulls. (If they care about logs to show a prospective static, that makes potions mandatory.) A few weeks later, at the peak of PF competence for the early fights of a tier, the prices for food and potions will have settled to about 2.5k each, and they'll have plenty of 100+ potion weeks reclearing P5S and P6S before they can hop into P7S prog for the week.
Capping tomes is no longer optional for them. Every week they have to make sure to do it, at least for the first ~8 weeks to get their BiS pieces, or they'll fall behind the rising tide of PF item level requirements. And they can't do it in Savage, because each of their reclears only grants 30 tomes.
They don't wind up touching crafted gear, because their tomes and Normal Raids have taken care of the item level 610 requirement for P7S by the time they unlock it. But between raiding itself and capping tomes, they're now spending 9-12 hours and ~300k gil (food, potions, melding gear as they upgrade) every week that they weren't before. That pushes out other content from their time budget, and they can't check retainers often when raiding - when a party enters and leaves instances won't line up well with venture times. The only way (within ToS) that they can make active money in this time is the uncapped tomes they generate alongside capped tomes, and potentially materia / desynth fodder from however they cap tomes.
Do you want to gear up your alts, therefore going back after your main job is done with P5S gear? 300k a week is over 1.2m a month is over 8m a tier even without farming during unlock, and if they clear P7S or P8S they add to that weekly figure. And that's before factoring in the fact that they probably make less gil than they used to, since basically every other combat activity is gil-positive with some diligence and they had to push something out. Suddenly the fact that they can sell tomes to offset their potion costs is a big deal - and so is the fact that they couldn't last tier.
That player didn't use any crafted gear. Now let's consider a raider who did.
This raider had their experience of getting their feet wet last tier in Asphodelos, or possibly beforehand. Their prior experience has polished their rotations and learning to learn, so static or PF, they expect to set foot in P7S in week 1. Their Normal Raid and tomestone gear will not get them to item level 610 in week 1, and they need to reach that item level before going in or they'll die to raidwides. (Seriously, even now healers need to know if someone's in 610 gear, because that character will need extra heals that the rest of the party at 615+ won't.) They need crafted gear - the only bypass to the current tier's weekly loot lock - in order to access the content they want to do, and they know it. Which means they may choose to get that gear earlier to have an easier time on P5S and P6S.
Therefore, they have a bigger first week expense spike, both from buying (and slot melding) crafted pieces to fill in their gaps and from having two bursts of "first clear potions" for P5S and P6S before the prices lower. A few weeks later, they've cleared P7S and are well into P8S, meaning they spend every week doing three reclears. Then for their prog, they face P8S' door boss every instance, drinking more potions in the hope that the party can get past it to the phase they want to work on. Unlike our earlier dabbler, this player is pretty much guaranteed to clear the tier on content (unless Earth constraints stop them), putting them at four reclears a week including a door boss fight. Their consumable costs still dominate the total; a highly consistent static that reclears efficiently might bring their tier-long costs below the dabbler by reducing their consumable use! Though if that static does optimization runs after reclears... imagine drinking 2-3 potions every pull for 6+ hours. Or 4-5 potions per pull in late Ultimate prog. I've done bulk food and potion orders for Ultimate raiders, and I'll do so again, because they go through so many potions.
(We've still not considered anyone who actually has a strong reason to overmeld their crafted gear. That's reserved for people who'll hit a fourth floor enrage in week 1 of the tier, at latest week 2, before tome and Savage gear makes the DPS check doable without it. Plenty of Ultimate raiders don't clear Savage that fast.)
You say you don't have a set schedule, which means you probably don't reliably have Tuesday evenings open, which means you have basically no access to statics - and would have an even worse time of PFing the tier than I described, having to endure the lower clear rates of non-prime-time parties. Does that sound cheap and easy to manage to you?
Having expenses high enough to struggle with doesn't require anything mystical. It just requires a fun profile slightly different than yours, that promotes raiding high enough to go for it despite limited time. That's what Amarande meant with