Supply and demand when it comes to materia and relics isn't a 1-1 Ratio you have to keep in mind it's more of a 1-2 ratio in terms of what can be supplied as well as the variable to even obtain a grade 4 materia. Using my example from yesterday let's look at belts when it comes to materia it's a 4/15 chance to getting the proper materia then from their only a 1/3 chance. If we wanted a more accurate fraction you could say there's only a 4/45 chance of obtaining x materia IV which is only a measly 8% and then to obtain the materia you "need" is a 1/45 which is a 2% chance of obtaining. Now on Gungnir from what I've seen "Dated Tarred Voyager Belts" are the best way to go since they are massed produce most likely for skillup and give you the chance at 4 materias which are used for relics Heaven Eye, Hell Fist, Touch of Rage and Savage Might, with current pricing they go for 1.1mil/450k/500k/750k you have to keep in mind though when it comes to belts there's only a 2.2% chance of obtaining these so basically 1 out of every 45 should grant you said materia.
Now the belt I mentioned before is 13k a piece so if you have to buy 45 belts to get the materia you need that's 585k, of course if variables worked in your favor and you managed to get all 4 of the above materia once in that 50 materia creation then you make some serious bank but more often then not you are given materia you can't use or isn't that useful. So let's say it is 585k to get wheat you want, then you have to take into consideration the time invested in spiritbonding the materia and of courrse the demand.
So you have something like
Maybe 1 belt an hour, I honestly don't know how long it takes to bond in a party it's most likely quicker so I"ll say 15-1hr
15=50 belts bodned in 12.5hrs
30=50 belts bonded in 25hrs
45=50 belts bonded in 37hrs
1h=50 belts bonded in 50hrs
You have to remember supply isn't something that happens in seconds it takes time to obtain the supply and then the demand will of course answer with their wallet.
Another variable Demand has is the aspect of the worth of said item that it's used for. Even if the demand was 1000 materia and say 100 people were interested in relics you have a situation in which say best case scenario everyone gets double meld in 10 materias. So 100x10=1000 materia used up by only 100 people.
Say you can only get a grade IV that needed for relics every 2hrs and maybe 30 people are doing it for 10hrs a day. So 5 materia each is gained by 30 people each which is only 150 grade IV materia. In order to supply that 1k quota those 30 people would need to invest 70hrs to a supply which could easily dwindle down in a matter of hours based on the persistence of the people interested in it.
Because of the % of obtaining a Grade IV, the time it takes to bond, the worth of said Relic as well as the amount of people who want it the pricing is going to stay high until there is something that's worth more.
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For one last fun thing based on Gungnir's pricing the following is can be said:
Belt=1/45 chance to obtain materia you wanted with 585k investment
Hands=1/27 3.7% chance with best case is 45k a piece so a 1.2mil investment
Head=1/30 3.3% chance with dated walnut 10k a piece so 300k investment
ring=1x2/21 4.7% chance with if 10k a piece 210k investment
Bracelet=1/21 4.7% chance with if 25k a piece 525k investment
So ya a lot of gil is required to arrive at that point and even then it's a risky business since even with all those attempts you may not even end up getting any grade IV materia you can sell to the MW.
5.5%:
THM1=1/18 50k=900k
4.7%:
THM2=1/21 70k=1.47mil
CON1=1/21 45k=945k
CON2=1/21 50k=1.05mil
4.1%:
PUG=1/24 50k=1.2mil
MRD=1/24 70k=1.68mil
LNC=1/24 90k=2.16mil
3.7%:
GLA=1/27 60k=1.62mil
ARC=1/27 90k=2.43mil
So ya if anything should prolly be glad with the current pricing, because of their usage in relics as well as other materia which is just naturally useful the chances of obtaining them and the potential gil needed to obtain it as shown above shows why it's worth as much as it is. That's not even taking into account the additional variables materia has but it's not needed here because when it comes to relics you can use any grade IV of the proper materia but for those who want the best of the best stats a 1/45 turns into a 1/180 which is only a 0.5% to obtain the best grade IV materia that you want lol.