There were 712 made between 1975 and 1977. 100 US version GTBs were made in mid 1976. The fiberglass bodies (glass-reinforced plastic - Vetroresina) were extremely well made and have held up well over the years. The front bonnet is aluminum and the rocker panels are steel.
The US version GTBs are wet sump with two distributors. The rest have dry sump engines with single distributors. All are, of course, non-catalyst carburetted cars. The US version cars have two air pumps.
A good source of information on the Fiberglass GTBs is the Ferrari Market Letter. They have two reprint articles available on the Fiberglass 308 GTBs.
I have been in the 76/77 GTB market for two years - finally found the one. In my area (Connecticut/NY/Mass) EVERY one I have seen that SOLD for $27K-$28K needed some sorting out mechanically, not in the best cosmetic shape, modified, missing books, history etc. These buyers wound up putting another $2K-$3K into them. Concours, low-mileage cars were selling between $34K - &35K.
Mine is an honest-to-goodness all original (except paint), intact example. I paid a little more for it because mechanically is was completely gone through (carbs, ignition, belts, hoses, brakes, etc), rare color, rare interior (no radio installed)and met my expectations in this way. The paint is by no means show paint, but it looks great. Because there are few good early GTB examples out there, I have found there is always a trade-off.
Agree with Sean and Anthony, when it comes to 308's there is a lot of CRAP out there...and yet owners still ask for top dollar. If the car is what the owner claims, it is probably worth low 40's, but you would need to be able to verify it's originality and condition very carefully, and get a knowledgeable person to look over the car. Regardless, this is big bucks for a 308, and you want to make sure you are getting what you are paying for. Me? I would have a F-club expert look it over, then have a body shop guy inspect it...if all looks good, then have the PPI done. You'll end up spending $500 to $600 to do all that, but worth it to me.