What's so good about old basses? Do you mean Vox, Burns, Hofner, Ampeg, even old Gibson basses (Ricks the possible exception)? Nope, it's straight into the price of 50's Fenders! Why are they expensive? Fender pioneered the electric bass, everything else followed!
If you want to spend you dosh on a 60's jazz or an Alembic that's your choice, but their pricing is always going to exclude a lot of us (me included). Your not going to find either going cheap. That's just the way it is unfortunately.