They put a comment at the end of the auction description telling people to come here and make their guess!
"On Feb-16-08 at 19:57:50 PST, seller added the following information:
If you log on to Blowoutcards website - you can enter their contest of guessing the final listing price of this Thorpe card! They are giving away a box of football cards to the one who is the closest. No purchase necessary and this is in no way an advertisement for them. This is just another testament of HOW BIG this card really is. Look at some of the guesses. $20K ++++"