Way more information needed!
Someone interested in one of your shots.....for what purpose? Do they want exclusive rights?
When you say you want to keep all rights, what exactly do you mean? That you want to be able to sell it again and again? If so, then you can only sell it royalty free.
http://photographersindex.com/stockprice.htm
That's a rough guide to start from.
Things to consider....
Is the shot outstanding...a one off....unique? If it is, then it's worth much more, and you should try to lock them into exclusive usage and charge a lot for it.
If it's fairly run of the mill, and if they could find a similar shot through any online stock agency, then play a little different. You may need to settle for less money, but then think about opting for royalty free, in which case you could sell the image again and again.
Pricing images is a bit of a black art, which is why I let a stock agency handle mine!