The value depends a lot on the condition of the components and what you expect to upgrade/modify.
In general, the steel Trek 8xx series were good frames, and if I needed a bike, I would be quite happy cruising around on one or modifying it for commuter use. If all of the components are in good shape, the set-up is how you want it, the bike fits you, and you wouldn't need to do a thing to it before you started riding, then I'd say $75 is a fair deal. If the components are in good shape, but it's not quite your style/you want to change the handlebar, seat or pedals or something, then it might be worth $75 to someone, but not to you. Components take the price down a lot, and quickly, and have a lot to do with your resources. I could build most of a bike from spare parts, and for me, I'd pay $20 or so for a good steel 80s MTB frame, like a Trek 800, if I was in the market. I'd modify the price based on condition and color and the weather and which side of the bed I got up on. If I was buying parts, I wouldn't pay much at all for something needing any significant help, not at this level.