I don't remember when they quit making the brown marbles, but it's probably older than that. Don't despair though. The brown marble was the best built. They play slightly faster than the newest tournament models but certainly would be better than a department store brand. Just the fact that they still play well after all this time is testament to how well they were made.
The market you're in would determine what a fair price would be. They'll bring more in the far Northwest than they would in the Northeast for example.
More important than the bar would be the playfield. Make sure the playing surface and sidewalls are not swelled or warped.