Depends on the asking price, the horsepower for that machine, and the number of grinders for sale in your area. I'm just up the coast from Los Angeles, so there is a ton of industrial surplus around here. I have a collection of grinders, so I'm not looking. If I was shopping for one, my ballpark would be around $75 -$100.
Factors to consider: Made in America, heavier than current machines, lasts forever, better balanced, likely larger bearings.
Negotiation points: Missing Spark Arrestors/Tongue Guards, missing coolant cup, condition, check to see if the feet are intact, condition of the cord, horsepower (is it 1/4, 1/3, 1/2 HP?), logos missing.