If that trim plate on the firewall is stamped 66-23717, it is a 66 Lemans.
From your description and the photos, I would not pay over $2000, and that assumes the frame, floors and undercarriage are clean and have minimal rust.
It's a LeMans; well restored, it might be worth $20,000. It's going to cost much more than that to complete the car, so I think you're looking at a "resto-mod" car. In that case, you might get a lot more for the finished vehicle, but you will also put a lot in it.
I assume that's a 326 engine under the hood. If it's a 389, I would pay a little more.
I'm sure others will chime in with other opinions!
Here are a couple of sites that can be helpful in determining what engine, transmission and rear axle are in the car:
Wallace Racing-Drag Racing Pontiac powered Firebirds,Trans Ams and Dragsters
The engine, head, manifold search functions are great; scroll down the main page and look at the links on the left side of the page.
VIN and Trim plate decoders, excellent photo resource
Use this site to see what the "maximum" market values might be, if the car was being sold by a dealer. Note: Low Retail assumes a running, driving car in need or TLC. This car would be worth less than half of the Low Retail amount, in my opinion.
Cool find; good luck with your decision!