What is "some rust but not bad"? One those cabs, there isn't much support when rust gets bad in key areas. I heard it's common that when the floor and rockers get rusty, you can replace them, sure, but then there is a good chance of the windshield posts twisting and then it's nearly impossible to get the doors to fit. I know a guy that was building one and it took him 3 tries to find a cab he could get the doors to fit on.
Without pictures it's impossible for us to tell you a decent value, and values can change a lot over different areas. I would trust Justhorsenround's opinion as he's closest to your area.
Are you set on a '69 GMC or is it just any year truck you're after? If you're dead set on that truck that may help sway your decision and value, but if you're not then you can wait for a better one to come along.
