If an engine is out of a car and you do not personally know the vendor or his real reputation it is a crapshoot in general. Also a good engine to one party may be less than that to another. As mentioned a fairly high range even across the board compression test is going to be the minumin requirement.
What does occur many times if engine is brought on the ground. It is installed and found to be a boat anchor. Does happen far too often.
Too many buyers out there are unreasonably optomistic. You have to do the homework one way or another.
Without it a better choice is to find a rustout or smashed vehicle that you can examine the engine running and test it. Then you really have a chance and might be cheaper as well. They are out there.
Half the fun or frustation of this hobby depending on your viewpoint is finding what you want. I did not forget your original question either. The fair price depends on availability and condition. You feel availability is scarce. This may be so. Condition though is not subjective at 1000.00. In my opinion it has to be really good if not almost exceptional to bring this kind of money. Just my opinion though.