Client wants my source material, but IMO, he didn't pay for it

I just finished a small project for a client, which earned just over $2,000. Along with the finished project, in a recent phone conversation the client also asked for "all source materials, in case we want to make our own changes later."

I'm not comfortable with this. For a small project of this size, we didn't sit down and agree on what the deliverables would be before hand, and there certainly is no contract. It's just an assumption of his that he own the rights to all my .aep, .max, and .fla files. In my opinion, he's paid me for delivering the final project.

So a few questions for you more seasoned freelancers:

1. I've been doing freelance stuff for about 3 years, and have never had a client ask for the source materials before. How common is this in other people's experience? Has anyone ever asked this of you?

2. How do I express that I don't feel it's in my best interest to hand over all my source materials, and back up my position that I'm not willing to do so while still remaining professional?

3. If the client were interested in paying for them, what price would I ask?

I'm not really worried about litigation or anything; if I dig in my heels and don't want to deliver the source material for free, the worst that I suspect is just losing this client (which is not a huge deal for me.)

Thought? Experiences? Advise? Thanks in advance

Todd