V7N did a contest before with a $12k top prize. GetUpdated gave a brand new car in 2007 as the top prize. Busby SEO Solutions of Australia did their contest twice, both with $5k as the top prize so they have given away more than $10 already.
I think the right sponsor that will know how to make the most out of the contest would be more than willing to give a large prize.
Will, You have told that you just didn't think that the web 2.0 websites will rank on top for keyword for long...but my "Review on RefererX" slide www.slideshare.net/guestcafa86/review-on-refererx on the 3rd position while my blogspot entry which was on the second position have now dropped. The other proof is that my sulumits retsambew webslide is ranking on 5th position after 2 months (I mean almost 2 months).
I am expecting more contest on the way. wwe_fan don't forget to invite me to your SEO Contest.