You'd do better to reward yourself for behaviors (which you control) than for weight loss (which is a bit less linear). Otherwise, you are going to run into weird weeks when you haven't lost weight but you KNOW it's water weight because you are about to start your period or whatever. Over and over and over again in this site, you'll see that the successful ones were the ones that regulated behavior, not weight loss.
I'd set yourself a set of weekly goals--a "bare minimum" amount of calorie restriction and exercise per week. If you do exactly that, you get nothing. If you do "better" than the bare minimum--skip a weekly treat you have planned, or do extra exercise--you pay yourself. If you do "worse"--skip a planned exercise or over eat, you pay in by some proportional amount.