..So what your saying is test her commitment or resolve with this contract. Force her to put her money where her mouth is. I'm not entirely sure how the prorated thing would work....what do you think about mileage? or is that just pushing it. The problem with contracts is enforcement ....I've paid enough lawyers bills for a while that's the downside to your idea.
Actually I was being sarcastic at you, but if you do decide to go that route, make sure you save all your receipts.