If you want to make a good hat site, work it on a script to simulate a well lubricated exchange market. Then, all prices will be accurate because they will be determined real time in a market system- like a simplified version of our own stock market.
The best way to do it would be to script it into graal. With the hat market NPC, players could place their requests and sales of hats while the price list is modified in real time. It would make hat trading much more exciting- and, unlike models other people have made for "stock markets," it would actually work.
