Might of missed it, but on test server the other day someone stuck out an idea for a fixed price and limited number per month, or week even.. sounded fairly good, the idea was that some people have billions to spend on those and they will buy them up cheap, and then re sell them for a higher price or if they find they from killing they will sell them for a high price, making it impossible for the lower levels and some underpayed level 50's to get them, so there could be a fixed price of so much, but so the people with lots to spend dont just buy as much as they want a limit per week or month on how many they can get per character.
