Always lame when someone asks for the 3d files. If this wasn't part of the deal or job in the first place, or this hasn't been negotiated in the beginning, it's always trouble. I've had a very nice client who loved my sketches of a character, and he wanted to buy the intellectual rights for that. We agreed on a price, set up the paperwork and both parties were happy and content. I also had another client asking for a 3d source file, and if I didn't hand it over, their client wouldn't pay the bill they owed to my client(about 40k bill) which put me in a position I didn't really had a choice. If I didn't give them the file, I would lose a very good client of mine, and they didn't get their 40k bill paid by their client. After several of these incidents I decided to talk to a lawyer who specializes in copyright and they made a terms of agreement, which I always forward to my clients. This did cost me quite a lot of money, but I was really sick of those questions and being put against a wall.
Also, when you say 'additions' or alterations to your model, that is something you have to agree upon. If you don't want to have any alterations or additions to your design or model, they can't , unless they have your approval for that (and your approval of the outcome they produce). You could say you'll be happy to make these additions or alterations for maybe a bit lower price for them, or that you have no trouble fixing things for them to keep the same level of quality. When you give your source files away, who knows who's going to work on them and what they might do to it (and destroy your work). If you have a very specific style, this could be a bad thing for your reputation. Others might recognize your style, but maybe not the amount of detail they're used to.
IF you decide selling the files, make a very good price for yourself. When the client has the source files, they can make countless more things with it, visuals, animations, anything YOU could be doing for them at a fraction of the cost of the source files.
But to answer your question a bit more to the point (sorry, I get carried away when these questions pop up): it depends. Who's the client, how big is the client (only local, national, international), are you selling just the files, but not the intellectual rights, and so on. The sketch I talked about in the beginning of this post, went away for 25k (for just a sketch and the intellectual property). Another 3d model went away for about 6k, without the intellectual property. So it varies.
And again, always go for the 'no, I'll be happy to do it for you!'