MicrostockGroup

Somebody wants to buy the rights to one of my pictures.  It's not one I have on any microstock or is otherwise making me any money.  I like it but I don't love it.
I have absolutely no idea what I should ask! 
Any and all experience and datapoints would be helpful.

Thanks

Whether or not it contains any people and is or is not model released would also be something to factor in. 

hmm - interesting point.  I never even thought about writing an agreement (yikes!)  I have a bunch sitting somewhere on a computer but where, oh where.
Btw, there are no people on the picture.
Thanks for the input so far!

I've sold 3 images outright (or 2 definite, one I'm currently negotiating)
Prices were $400 (this was a stock image with few sales) $1500, (a more popular image but with under 100 sales) and I'm currently in negotiation with a major beauty company for another for $3500. This is a simple image but with sales in the many hundreds. In the first 2 examples the companies involved drew up contracts acknowledging the rights of previous buyers but which granted ownership to the buyer, the last one is a bit tricky as the companies legal dept. are balking at the fact that so many have already bought it and I may have to re-draw the image ( for which they are offering extra money for a custom artwork)