The price is oк. That's an average price for renting signature. For exampe, if you'd like to place ad at this forum officially, you'll have to pay 100-150$ for 120x20 banner which rotate or 700$ a week for a big banner at header.
Look at the prices:
http://www.armadaboard.com/pr.htmlJesus Christ! You're asking for signature space on your account NOT on the forum's ad space.