Exercise motivation for friends (edited to clarify)

Tiffany
My good friend and I are trying to come up with a motivational incentive to keep us both on track (time to loose "mommy the tummy). 
A while back, we did, the one of us who did the most exercise in a week had to buy the other person lunch, at our weekly lunch meeting. 
But that didn't work because she has more financial wiggle room than I do and after I lost a couple of weeks, she felt like it wasn't fair to me. 
Any thoughts, ideas, or success stories?