wlongacre - 7:15 pm on Oct 6, 2010 (gmt 0)
You didn't say if you are making an income from the site. If so you should factor that into the price.
Also if you are so inclined, you should view this as an opportunity to make some money. You've got a site that has a rank of #1, that alone makes the site worth a good deal more than what your costs have been. Not to mention the many hours of personal time you devoted. I think you may be sitting on a site that should bring 10k$ or more. If I were in this situation I would start with a high figure like 25k$ and negotiate the price.
No matter what the price you choose, make sure you document all aspects of the deal (in writing) and use an escrow account to transfer funds.
Best of luck.