I just sold a 1990 Diamondback Master TG with a full 105 groupset for $210. It was in really solid cosmetic and mechanical condition- everything was properly greased and cables were in good shape.
I think the buyer got a pretty good deal on it considering the size and time of season.

I hated selling it because it was a really strong ride- it wasnt harsh but it felt stiffer than most older road bikes due to the OS tubing. It was just a bit too small in the end.

The Prevail TG was a step up over the Master TG and thats evidenced by the 600 components instead of 105.
If the Prevail TG you are looking at is actually a '93, then it has 600STI shifters. Assuming those work properly, I would pay $300 for it as a keeper.

I absolutely would have put $350 overall into my Master TG(bought for $200) if it had fit by swapping out the shifters for Microshift 7sp STIs, new cables and housing, and new tires. That is comparable to if you buy yours for $300 and make updates.
You will most likely need new tires($50-75), new cables and housing(probably rusted), and new bar tape since the bike has been hanging up unused for almost a quarter century. Fresh grease in the headset, hubs, and bottom bracket would be great too, but probably not necessary right away and could wait until the winter after you got some miles in.