Hard to say really. It could be just a bad switch or it could be a loose wire. I would wager the cost to repair it would be minimal. To me, a 'super' price for a TSR-8 in good shape would be anything under $300.
What actually happens when you press top? Does the machine try to stop but fails? Does it just not respond to the button being pressed?
You could consider looking for a RC-408, the remote for the TSR-8, but getting the button on the reel to reel fixed will likely be less expensive. The remote is great though and something you may want to keep an eye out for...
Robert