I actually find this really interesting being at the end of a macro economics class this semester and learning about supply and demand from a non basic stand point. From the date of 2/6/19 to 3/16/19 I made a spreadsheet in excel to track market price fluctuations every Saturday and found some really good data cause it was in the time of the lunar New Years event and the boar limited items. I don't know how to link a spreadsheet in this reply but when the boar costume was released, the market became more expensive in all items listed. This would be because of RP "prices" (players wanting to spend in game currency rather than real $ for RP) making a demand for itself for people to buy the new items. Because of the RP "prices" going up, this caused all items, regarding RP, to go up as well. I don't know the exact time period but at one point, a 7 day donator pass was 300k, compared to, the Saturday of the week of boar release, at 1.2m. The weeks after all prices have steadily gone down. The best solution to an economy is to do nothing cause everything will be paid for the prices it's valued at based on supply of the item and the demand for who wants it.
An example of a damper on this MMO economy would be re releasing an old limited vanity. If the kyu hat (which I've been trying to get for years) was release at 1000RP or in the mystery boxes again, a huge inflation would take place because the kyu hat would basically be worthless compared to its current value and wouldn't move millions of yen elsewhere.
Im no master at economics, or would be able to make any huge decision to change an economy, but I believe that money needs to be taken out of the game if you want to see the stabilization at whatever you're looking for. A small solution would be to get rid of alternative accounts who berry farm and flood the economy with money, this could be seen as like counterfeit money destabilizing an economy. I don't know how it would be done if someone has more than one device that can play the game and get around a one account per device system.
The undercuts don't do anything to the economy because the potential moves in a flat line but the real market goes up and down slightly anyways or items return to their "true value" after an inflation or recession period. For the admins to fix another 300k 7 day donator situation, all they'd have to do is release something that people want in the store (limited or not) and it would fix the "price" of RP and the donator or any item similar would go up. It seems contradictive to say "do nothing" and then proceed to say this but at times, it's alright to give the economy a little nudge if you want it "fixed" faster. In short term, in a recession (not enough money), you need to put money in, and in an inflation (too much money) period, take it out. In the U.S. government, this is by buying and selling bonds (a promise for money at a certain time) most of the time, or sometimes a change in taxes, depending on the policy you're using. I don't know how to correlate this to MMO unless NPC become more important in buying/selling items, and even that isn't a promise for money like a bond. Listing prices would be made higher or lower depending on the market situation too.
If anyone wants to see the spreadsheet, tell me how to make a link to an excel spreadsheet and I'll reply to this with it. A photo would make the graph too small cause the amount of items that I tracked.
Edit: Images of spreadsheet (not wide enough to stretch to 3/16/19)
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