Working, anywhere from $350-550. Depends how badly someone needs it. Shipping is extra.
^ That is well below market value ATM. Quoted price is what members here would
LIKE to pay, but the reality is nearly 2x those numbers.
Working, low burn 6100 - you'd sell it in a matter of hours for $750.
To test it safely, you can unplug your Tempest PCB from the cabinet (remove edge connectors) and use an extension harness or simply plug the unknown monitor in your current monitor's place. You can then power it up with no video signal - just power - and verify if the HV comes alive and the spot killer goes live on the deflection. If so, you may then reconnect your Tempest PCB and proceed to fire it back up with video signal.
However, it is unlikely working. If it were, the person who sold you the game would definitely have mentioned they are including a working spare. In all likelihood, it went down and was replaced with the one in the game.
Of course, even not working or untested, you'll likely get $500 for the complete monitor - more if sold piecemeal as tube, yoke, boards, etc.
.
WANTED!!! : Robotron Cabaret - NeoSaveMasta - SD->MH Adapter & Spinner CP