Most of the stock photo agency websites have built in price calculators which can give you a rough idea of what you might want to charge (ie Getty, Corbis, AllCanadaPhotos, etc) but you'll probably want to ask quite a few questions about the intended use by the buyer. You'll need to decide whether to you want to sell as Rights Managed (RM = specified, limited/restricted usage and duration) or Royalty Free (RF = generally unspecified, unlimited/unrestricted usage and duration), and you'll probably want to ask the buyer quite a few questions about how, when, where, what size, and for how long they'll be wanting to license the use of your images.
Once on the stock sites, select a similar type of image, choose "get a price" and start filling in as many of the parameters as you know. For RM there will likely be many specific parameters, and for RF there might only be a pixel size list of parameters (ie large size for print, and smaller specific sizes for web use).
For the record, I've never sold any licensing to anyone. Anytime I've been approached, and found out the specifics, my price (being RM) has always been too high. Everyone always wants RF and to pay pennies for them...
Good luck.
