Well, in the condition it's in, that's a fair price. Not good, but fair.
I would try to talk him down a little since it is a four door. If I were
you, I would hold out for a two door (unless you want a four door). You
are never going to get your money back out of a four door renovation.
I'm also kind of lost on the "no rust, but floorboards need to be
replaced" part. If those are rusted, you need to look inside the trunk.
Right were the floor seam along the rear near the bumper. This is a bad
spot on these cars. You'll probably find some there, too, and that can
help you talk the price down. Don't forget to crawl under and feel the
fenders on the inside. Fuel tank might be pretty shabby, too. They
aren't cheap. Check along the chrome trim around the windows for bubbles
in the paint. That's rust.

What you need to look for first is to see if the car is complete. What's
missing, and what's it going to need. Some parts can be brought back to
life with some paint and elbow grease, others will need replacing. Make
a list of what's wrong (glass, brakes, rubber and cloth door and window
seals, interior, motor, rear axle, chrome, etc. Get a catalog from one
of the many on-line companies, or check the web for the price of those
parts. Chrome isn't cheap. Then you will have an idea of how much it's
really going to cost you to get the job done.

I harp on the money issue, because of the cycle involved on these cars.
We buy it, fix it up, hang on to it for a while, then sell it to buy
something we want more. It's a lot easier to buy what you want if the
car you finished is worth something, hopefully what you put into it.
Don't ever expect to get your labor back, but don't sink 10 grand in a
car that will only be worth 5 when you are finished. That's what new
cars are for. IMHO, of course. A lot of people jump in not knowing what
a money pit some of these fixer-uppers are going to cost them. They turn
around and sell them for a huge loss to someone else. Start with a good
foundation.

If it's a car you truly like, go for it.