Okay so they weren't selling as a buy it now, so let's go with an auction.

Auction Style Bidding
Any Species, No Wings, SFW Only, small adjustments can be made with additional charge depending on change.

Bidding ends This Saturday, June 21st, at 9pm EST Time.

Bidding for each spot begins at $10, minimum increments of $5.
-No Bid sniping, any bids made 10 minutes before the end will extend it another 10 minutes.

$10+ earns you flat color. ex: http://www.furaffinity.net/view/11124016/
$25+ earns you simple shading. ex. http://www.furaffinity.net/view/12076915/
$40+ earns you cleaner shading/detail. ex. http://www.furaffinity.net/view/12137140/
$50+ earns you an additional 'pencil' sketch. ex. http://www.furaffinity.net/view/12062696/