I did a compression test on an engine I am interested in and got 100 psi with a standard compression tester. Is that good, bad or so-so?

It starts right up and does not smoke. It's in a IT7 that has about 20 races on it.

What is a fair price for it?

 

Compression number sounds good. I assume this is using a piston
compression tester while holding the button down or modified and
looking at the spikes while cranking. If not then its hard to say.

I have no clue, probably depends on condition and spares that come
with it. You might be better off posting this same query in the race tech
section, more racers there that may see it and respond better.