I gave pre-HQ advice for exactly this question here. However, some of it doesn't still apply, since T4 spam seems like it'll be much harder than when everyone was grinding light, and PvP has been extremely hard to get into with everyone busy doing the new content. PvP will be easier to queue for soon when the new PvP rules and gear come out next patch.
At 85, you have two main options right now: Use new stuff with really terrible chances, or use old stuff that might be cheaper than it used to be.
For new stuff, at 85, with Tinker's Bacon (HQ Tinker's Calm + HQ Bacon Broth), you should have a 15% chance to desynth Agate Rings (r115). I have no idea how much exp this would give, but let's assume 0.9 (it could be higher, I dunno, I've never heard of someone trying this). If you meld materia, every materia gives +5%. Agate Rings cost at most 10,949 (vendor price) each. 10949/(0.15*0.9) = 81,103 gil per level unmelded. Let's assume the cheapest materia you can find is 500 gil (a high estimate, I'd expect). ((10949+500*1)/(0.20*0.9)) = 63,605 gil per level for a single meld. Better! Now, double meld: ((10949+(500*3.22))/(0.25*0.9)) = 55,818 gil per level. Triple meld: ((10949+(500*7.39))/(0.30*0.9)) = 54,237. We've hit diminishing returns. But, if you can get materia for cheaper, like 200 gil each, we haven't yet (46k for triple meld, vs 51.5k for double meld). So, if the cheapest materia you can find is 400 or more, double meld. If the cheapest materia you can find is under 400, triple meld. Obviously, get all the rings and do all the melds until you have ~25 prepped and do them all at once to save on Tinker's Bacon. Without Tinker's Bacon, this is all over 100k per level and I wouldn't recommend it. With NQ broth/calm, you're also looking at a sharp price increase. So, take those into consideration. I have no idea what Tinker's Bacon costs on your server, but on mine, Tinker's Calm had been cheap for a while since the mats it requires are used by absolutely nothing else. I think I bought the mats for 50 of them for 50k total.
For old stuff, check out what's happened to Rose Gold Ingots and Gold Ingots. All my Weathered Soldiery items converted into Rose Gold Ingots, so they might be tanking in value. Also, you can now buy all the Soldiery mats from the GC vendor. Since most people expected a new GC rank, I'll bet most people did what I did and saved up 50k GC seals pre-expansion only to find that not only are there no new GC ranks, we're getting tons of GC seals from GC turnins for crafting leveling, so we need to dump our seals fast. I'll bet all GC mats are tanking... though some are also needed for airships, so that may go up instead. It's hard to tell. On that note, Rose Gold Ingots are also needed for airships, so they might and might not be going down.
Definitely check melded Gold Spectacles. If you can make them, check both the price of the finished mats and the base mats. Do the math out - every server is going to be wildly different for those mats right now until the markets settle. Here are the formulas to plug prices into:Remember, if you have access to Tinker's Bacon, increase the values appropriately (so like, in the first formula, the 0.40 becomes 0.55 with HQ Tinker's Bacon).
- Unmelded Gold Spectacles: ((53+{Wind Cluster}*2+{Fire Cluster}+{Rose Gold Nugget}+{Gold Ingot}+{Terminus Putty}*2)/(0.40*0.5))
- Double-melded Gold Spectacles: (({Cheap Materia}*2+53+{Wind Cluster}*2+{Fire Cluster}+{Rose Gold Nugget}+{Gold Ingot}+{Terminus Putty}*2)/(0.50*0.5))
- Triple-melded Gold Spectacles: (({Cheap Materia}*4.22+53+{Wind Cluster}*2+{Fire Cluster}+{Rose Gold Nugget}+{Gold Ingot}+{Terminus Putty}*2)/(0.55*0.5))
- Quad-melded Gold Spectacles: (({Cheap Materia}*8.39+53+{Wind Cluster}*2+{Fire Cluster}+{Rose Gold Nugget}+{Gold Ingot}+{Terminus Putty}*2)/(0.60*0.5))
- Carbuncle Chronometer: ((2570+{Wind Cluster}*2+{Fire Cluster}+{Rose Gold Nugget}*5+{Gold Ingot}+{Cobalt Rivets}*4+{Ruby})/(0.40*0.5)) [remember to calc the value of a finished mat using the base mats if that's cheaper, like instead of {Ruby}, ({Fire Cluster}+{Wind Cluster}*2+{Emery}+{Raw Ruby}) might be cheaper]
- Furnace Ring: (({Wind Cluster}*2+{Fire Cluster}+{Basilisk Whetstone}+{Belah'dian Silver}+{Aged Eye of Fire})/(0.40*0.5)) [Remember you can replace "Aged Eye of Fire" with 15000 (average price of getting it yourself from Aged Rings) if that's cheaper]
If you work out what's most economical for you, please let me know!