Why would his attitude on the tape affect his wanting it? Even if he was negative/harsh/cruel on tape, wouldn't he want it, even if for no other reason than he wants to prevent the public from ever hearing it?
Again, he also would pay more than a lot of fans would, I'd imagine. Institutions such as libraries and colleges may want it, such as Williams College (his alma mater), but I couldn't see them being able to pay much for it.
If your only goal is to make money, your best bet is Sondheim himself or an avid collector/historian who has the money to offer that you're looking for. How much ARE you expecting?
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle