Well - the best I can come up with, given the info you've provided is - it depends. It depends on what kind of sponsorships these are, and the medium of the sponsorships (is it an online situation, for-profit, or for a charity via a sports event?). Is your geographic footprint local, regional, or national, and what kind of marketing materials are you creating to support their efforts? What is your cost for these sponsorships? And since $10,000+ bucks is a fair chunk of change, I'm betting your prospects are mid-size to large companies, who may be seeking innovative marketing opportunities. These folks are ina tough market to engage with, and the turn-around time from contact to sale can be many months. Even without knowing the answers to these questions, I believe a salesperson working on a commission-only situation should be very fairly compensated. Once you have these factors figured out you can create a fair rate... In absence of those stats, I'd suggest something in the neighborhood of 20-30% commission.
Here's to you winning in business!