pay for what you'll use
Like others have said, it is very tough to determine a fair price without seeing it. What you need to remember is to pay a fair price for what you will be using, but not pay for a lot of "gennie" stuff that you won't use and may end up throwing in a dumpster. For instance, if you know that you are going to use different wheels and a different front suspension, the fact that they are all there is of little consequence. Go ahead and take the entire truck of course, but you'll probably not be able to get anything out of the wheels and original front suspension (except maybe scrap metal prices). However, trim, door handles, and a lot of that type stuff gets expensive in a hurry if you have to replace it.
If the body is straight, it is amazing what some media blasting can do to clean up sheet metal. If this is indeed a five window GM truck, it is probably one of the Advanced Design trucks, which means that patch panels are available. So, starting with something is better than nothing.
Even if just the frame is usable, they are certainly more desirable than the aftermarket chassis that are available. The original frames have some graceful lines, while the "repro" frames are purely straight with angular cuts in them.
Just food for thought.
Dennis W. Parks
Author of automotive how-to books