Prices for Firebirds seems to be all over the place. I just moved back to Kentucky from So. California and the prices here for first generation Firebirds seems to be higher than on the west coast. My recommendation would be to check the on-line auctions and recycler.com to get some comparisons. I am sure that you will be getting some hits from the FGF BB about your car.
I have been looking for a decent '67-'69 convertible that I can restore and I am seeing them as low as $4,000-5,000 and as high as $16,000-18,000 with some of the lower priced cars looking better than the higher priced cars.
To me, the more original parts that are available the more value. I am seeing some pretty nice "looking", high priced Firebirds, only to find out they are pretty far from original and I would have to spend money/time to get them back.
I have been in your situation several times in the past and have always regretted selling my cars, but you have to do what is best for you and your family and hope you can find another one in the future when you are in a better situation.