Your "payment" method for losing weight is admirable.
It seems like it'll actually work because its an added incentive. I would opt to follow suit, but I don't keep cash on me and I live in a tight-spaced college dorm filled with broke students and I would fear my money collection might get stolen. So here is my (somewhat similar) method of "paying myself to lose weight"...
I do a simple Reward System. I wrote down a list of things I would really like to buy myself in the near future, but put a "price tag" on them in the form of pounds lost. For example, my total weight loss goal is 20 pounds altogether, but when I lose 15 out of the 20 pounds, I will reward myself with a new tattoo (I've really wanted one for some time now), or when I lose the full amount i will treat myself to a brand new wardrobe, and if i manage to keep the weight off for up to a month, I will buy myself the new Tilt 2, a phone I'm DYING to get. But my rewards are awarded after incremental losses of 5 pounds. So after I lose this first five pounds (I will weigh-in to check my progress every Friday) my first reward would be to splurge on beauty products that i don't really need, but would like to have (start simple).
I'm a woman who likes to have "stuff", so if i put a condition on getting those "stuff", I'm hoping i will be more motivated to lose this weight.
Just a suggestion for anyone who needs ideas!