'84 GN owner...
I paid $7K for mine back in 1996. It had options that very few '84/'85 cars have (Factory Moonroof, G80 posi, and Digital dash). I have yet to see an '84/'85 car with as many build codes as mine.
Was it worth that? In hindsight? No.
There are alot of FAKE '84/'85 GN's out there. Be careful. If there is no trunk build code you better run... not walk away from the car. You should expect to pay a tad more for the GN because that is what is desired out there. IMO though the T-types of those years are more desireable because nobody see's them coming. A GN is well known.
You will see these cars going cheap $3,500 on up. They are not valued like the '86/'87 cars. In many instances these cars require extensive labor to bring them back to daily operation, and the costs of a replacement 200R4 transmission, or a turbo, or engine work is just as expensive as that of the '86/'87 cars. Aftermarket parts are there to support these cars but they aren't cheap either. There is a low miles '84 GN that was on Ebay and the price was over $19,000. It didn't sell if I recall from memory.
If you want an '84/'85 because it is an entry car, or it is your goal to make a 13 second "Hot-Air" great! If it is your goal to buy it... and then flip it... you are wasting your time.