I haven't come upon this pricing dilemma before, but I'm sure its not uncommon. An agent I work with will be the listing agent for nine (9) townhouses and has asked me to come out and shoot one of two completed units. She intends to use the images for both listings and possibly the other 7 (as the finishes and configurations are similar) ...
I'm sensing she thinks she'll pay for the photos once and just keep reusing them. I think there should be a per use fee associated with each additional listing, but I'm not sure how to broach the subject or what that fee should look like. Any suggestions?
Your question implies that you've never explained to this client what the money she paid you is and how she is allowed to use the photos you delivered to her. Don't feel bad, this is an extremely common problem with real estate photographers.
The photo licensing discussion:
If you never say anything to a client about photo licensing they are likely to assume that they OWN the photos you shoot for them and they can do anything they please with them. However, the normal convention for real estate photography is that the money the client pays the photographer is for a use license to use the photos for a single listing and everything they need to do as a listing agent to market that listing and that their usage rights terminate when the listing sells or is terminated.
When to talk about photo licensing:
If there never is a licensing discussion and a situation like you describe arises then it is much more awkward to talk about licensing to the client than if the discussion occurs up front at the beginning of the client relationship. But from the situation, you describe it's better to have the discussion now than later.
How to talk about photo licensing:
As you'll see if you read previous photo licensing discussions here on PFRE, even though licensing is basically very simple. There can be a number of different issues and situations that can arise out of photo licensing so it's best to have a written photo licensing agreement that you can give them and have them sign during the discussion. Joel Rothman has a sample downloadable photo licensing agreement in his recent post. I also have Joel's sample agreement in my Business of Real Estate Photography e-book. Joel, I'm sure would recommend that clients sign an agreement for each and every shoot.
My recommendation on how to handle your current situation:
It is much better to discuss and clarify photo licensing with her before she decides to reuse the photos rather than after she starts doing it.