Zmud is auction based, and only happens once a year if I recall. This is from an email I received from them in 2008

If you haven't used the bidding system before, please be sure to read the instructions linked to the page given above. Bidding works similar to eBay, with a minimum starting bid of $1000 for a year of icon display in CMUD and zMUD. The top ten bids are selected for icon display. Last year the last winning icon was $1500 and the top winning icon bid was $3000.