There's not a way to put a solid price on what a photo is worth, imho. There's a lot to consider, so it's not easy to set a certain price for everyone/every photo. Here's a couple of sites I use for reference that might help:
The first one discusses some valid points about prices:
http://brianx.com/lifestyle/photography-prices.html
Here's another site with some helpful info, but you'll need to consider area among other things:
http://www.fotografi.org/fees_professio ... _ITALY.htm
Unless the photo can't be rephotographed, there's little to stop you from taking another similar shot, or reproducing another similar exposure you took at the same time (for that matter) depending on how the terms of sell are quoted. Also, will it bring you more business from the buyer later on, and what are the likely chances you'd sell if for other uses in the future? Could these people find another shot which is similar, with the same quality or higher? If it's of something that many people photograph, they may very well do just that if you price it too high. So be realistic, and do some additional research. Unless you have people lined up to purchase your photos on a regular basis, pricing can be your worst enemy in helping future sales.