This has been an excellent case study in the problems with free markets. I'm sorry I missed it! No matter what the solution, it would be great to track what had happened (lack of cash depressing prices), into the boom day of Christmas Eve, and now what to do.
I had written a long post last night, but decided to wait to post. I'm glad I did!
Here are suggestions I see for a long term solution.
1) If booster pack retail prices are 10 gold, that means that booster pack wholesale prices are about 5 gold. In real life, I used to buy boxes from a wholesaler for just under $2 per pack, which would equate to roughly 5.8 to 6.0 gold. The wholesaler made a small profit, so his prices was just about 5.0. Therefore, if the average rare price goes above 6 or 7 gold, the wholesaler should just open packs and stock the store.
2) I would also assign a cap of 99.99 gold for any rare, equivalent to about US$30 in real life. This is a little artificial, but it prevents massive trickery. Also, to the best of my memory a non-promo never got above $30 in real life. In Magic, everyone worried when Jace got above $50 - I'd like to avoid that.
3) On the low side, I would implement a minimum sell price of 1 gold for any rare and a minimum buy price of 2. In real life, there are always transaction costs. I will note that big Magic retailers like Star City games will buy any mythic rare for 25 cents, about 0.6 gold in our system. A mythic rare appears once on a Magic rare sheet, making the probability roughly the same as a rare from a large set like FotR. Therefore, I think 1 gold is a reasonable floor. It makes things less worthless.