While complete MTBs have started to slowly increase in value, an incomplete bike not so much.
I did recently pay $25 for a 1990 Mongoose in poor condition. But it was full Deore XT components. Since I was out of room (too many bikes picked up), I donated the frame, fork, and Deore XT wheels. I just kept the aftermarket shock stem and the remaining Deore XT components. So you could say I paid $25 for it without wheels. But the parts were desirable. A 1990s Trek 850 is not going to have desirable parts.
If I found the OP's bike in the trash I'd grab it and donate to the co op. Not worth the time, effort, and expense to make it complete, and no juicy parts to make parting out valuable. Maybe I could get $20 to $30 for it as is, but I am not going to deal with the C/L selling process for $20.