TF2WH can give you a good starting point. Except for unusuals, prices are determined automatically from supply and demand (price change list here) and your chances of landing a bad deal decrease accordingly.
The unit of measure there is a credit, a 4,500th of a refined metal. You can request prices for any item here. (TF2WH doesn't accept non-craftable items, for the record, and the prices reflect that.)
Just keep in mind that the prices are not necessarily aligned with what you'd get from users. The idiot box, for example, is priced at 900 credits, or less than two scraps... if you can find it there, given that apparently people are willing to buy an idiot box for a refined on tf2outpost. (Ugh.)