They're too rare to declare a "Blue Book" price the way you could for a used car. If more of them were changing hands, you could try to average out what they bring.
Better to think about what your other choices would cost. Can't figure it's worth more than half what you'd pay for a new amplifier that does the same job.
Besides, it's got to be thirty years old, more or less. Doesn't matter so much what it was like when it was new. How much of the last 100,000 miles' maintenance has been done? If it hasn't been, that expense will fall to you once it gets put back into regular use.
It's not just the miles, it's the years, too. Thirty year-old relays, filter capacitors and such may not have a lot of "miles" left on them. Can't just measure the tread depth like a tire, or look for obvious dry rot.
Consider also that it was built for a radio that's "small" by today's standards. There weren't any "dual-final" base radios in 1975. A small radio will probably suit it fine. A large radio may overdrive the thing and give it a headache.
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