Well if it's Royalty Free they would pay just once for the image and could use it for anything. Most people buying Royalty Free buy a DVD with about a 100 images. They can then use them for what ever they want with no more fees.
With Rights Managed images you find out exactly what they want to use if for and build the price around this. If they want to use it again for the same thing or an entirely different thing they pay again.
Both methods are popular today and make the agency lots of money.... the photographers not so much.
The problem with Royality based sales is the hassles down the road. The client thinks they have bought the photo so when they go to reuse it they don't feel they should be repaying. And if they're out of town or in another part of the world they could use the photo for years and you would never find out.
A third option that wouldn't really involve just a single photographer is subscription fees... a client pays a flat monthly fee and can use any photo in the collection for whatever they want. This is certainly the way things are moving and will probally be the doom of stock photography.
So what is your picture worth... try to get as much as you can, but since it's not the only picture available the client will have many options on price.
This is totally unrelated to how great your photo is... I happen to like the shot, and they must too, but it's not the only shot available to them.
Just my 2 cents worth.
-Rob Skeoch
www.bigcameraworkshops.com