Take how much you earn from it in a year, times it by two, and that's how much you should ask for it at an absolute minimum. Although that would be a 'short of money and could do with the cash' situation.

If money isn't an issue right now, then either don't sell it, or ask for five to ten times what you're currently making a year.

Remember also to factor in the earnings power of any sister / similar files you have to delete (if that's an issue).

I once sold the rights to my best selling illustration for the amount of my projected earnings for the next 7 years (a high four figure number) - the file has already been online for about 5 years and sold up to 10 times a day at SS (not much on the other agencies). Since the client accepted without hesitation I think I could have even asked for more.

Also sold exclusive rights for 2 years for a photo (and a sister image) for about 4 times of my previous earnings