I'd definitely look for 24x (or more depending on the potential buyer(s)) the money revenue of the website if it meets the conditions outlined above by Falian. If you have an established website that has been making this for months, and you can prove it, then it's easily worth that.
If you're making $1000, that's really good. The only reason I can see for selling it is that you need the money. If this site has been online for a good six months or more, I would sell for at least $15 000 if not more. On average you sell a site for about 10 times the amount you would make in a month.