Are they getting rare? Always. They dont make them anymore, and every one that gets wrecked, rusted out, parted out, or turned into a racecar makes less of them out there. (on the street anyway).
Good news is that they made them for a long time. 79-93 If your not stuck on it having to be a 4 eye or aero nose or even an original v8 car. Some people do the swaps "right".
Cars from the late 70's to 80's in the salt belt that have been driven year round are all but dissappearing. How many 77 oldsmobiles do you see? not many, and they made a MILLION of them that year alone.
Other cars that are all but dissappearing, Escort gt's, T-bird turbo coupes, probe gt's, taurus sho's, Ford exp's, the GMG-body cars (gn's, monte ss's, etc). Hell even the DSM (talon, laser, etc) cars are dropping like flies.
As for value on that gt. hard to say. I paid 4400 for my Lx 4 years ago, with more miles on it. BUT, I knew the owner, and it had a folder of reciepts of maintenance for the last 5 years totalling double what I paid for it. New top, carpet, seat covers, all maintenance, new exhaust headers, etc etc.
If its really really clean, yeah that price doesn't scare me. in the 3K range would be nice, but how soon do you want a car, and how far are you willing to travel? Deals do pop up, but rust free stangs that aren't hacked up or beaten to death in MI can be hard to find. Especially if you want a certain color or trans.