Charge whatever it take to make your business expenses, if you have any.
Producers are always asking questions like these, and it usually because they don't know what the can get for their music. If this is your circumstance, you should check out soundclick's music charts or myFlashStore.net to see what other producers are charging for exclusives.
Be honest with yourself and you'll find the right number for you. Some people may try to negotiate the price with you, so stand firm. Read mymusic blogfor other business related matters.