Hi Al -
I've had a few opportunities over the years to buy multiples of operators' grip testers- both Gottlieb and Mercury.
They were quite popular in their time and were in service for long periods. I've had several "batches" of the trade-stim variety.
I would not consider them rare, but they are certainly more desirable as compared to the basic Gottlieb models.
It's been over 5 years since I've sold one, so hard for me to give a value (esp without seeing yours).
I'd be happy to talk more with you directly - feel free to email me @
coinopgeek@gmail.com-John