How much the machines are worth depends on how hard it is to find one where you live.
I wouldn't pay that for one in sad condition - I have sold some in top condition though.
You can try offering her how much you feel you can pay since it is not in top condition.
They are a good machine a lot of people love them.
The 403, 503, are good dependable zig zag machines - Here is a link on the difference between them and the 401 or 500 which usually cost more:
http://www.quiltingboard.com/main-f1...s-t160607.html
Singer 99 or Spartan or 404 are my fav little vintage straight sew machines as well.
I really like my old 15 clone sewing machine. Here is a link to more info on those:
http://www.quiltingboard.com/picture...p-t164361.html
If you read through you will find plenty of info - these machines are frequently pretty inexpensive. Some times free. Some times they freeze up. Expect to pay more for one that turns, sounds good, and looks good. These are a simple machine - parts are available - manuals exist and they are nice to sew with.
Here is a link to a 15 clone I cleaned up:
http://www.quiltingboard.com/vintage...r-t169127.html
There are some real nice treadles out there inexpensive if you want to work on one - some 66s have back tack
Muv has some real good videos on the sticky for how to clean up a machine.
She does not have you taking apart a machine completely all at once.
There are some machines with long bobbins, some drop in and some have a bobbin case and go vertical.
Dig around on here - it could take a week or two to read all the posts on here but it might
give you plenty of information. J. Miller seems to have learned a lot in 3 months he's been reading on here about machines.
IMHO The new plastic machines aren't as reliable as the old ones.
It is nice when you find a metal machine that turns smooth, sounds good, makes a good stitch and looks fab.
Make sure the machine has a bobbin case if it is the type that uses one, a foot or knee control, tension all there and in place. Back tack all there. Bobbin winder work?
If the machine doesn't move, look for dried on oil or rust - offer less money when you see that - sometimes those will be free.
Finally, if you have any questions feel very free to ask. There aren't ANY dumb questions. You might even stump some of us - probably not all of us.... I can usually tell you where to look for information. So much is out there and so well written, I don't try to reinvent.