It depends...
This is a "buyers" market. That being said, there are lots of folks that have great cars and tons of money invested in them. No one wants to take a beating on selling their GN. As you noted, there's lots of cars in the $8k and below range. Sometimes, you'll find a steal of a deal, however, be prepared to get what you've paid for... (lots of cars with rust issues, paint issues, etc.)
Naturally, some people (ie: Buyers) see these cheaper offerings and assume that other cars should be equivalent. But, then there's the $20k+ range (most of these are sub 50k mile cars)... Again, you'll get what you pay for.
You need to know what you want specifically. Do you want a turn key car that has everything done? If so, expect to pay a premium. Do you want clean title or would salvage title work for you? Is this going to be a dd car, a garage queen show car, or used for drag racing?
For the basic estimates, figure it this way: $5k-6k for decent paint & body, $1k for weatherstripping, $1-2k for trans, $1k-1.5k for interior, $500-700 or so for decent tires,and that's just some of the basics without even mentioning the engine... and then there's the engine... Do you want modded or not modded? How many mods? Engine rebuilt or stock, P&P'd heads or stock?, bigger turbo? engine management?
All this stuff costs money.
In the long run it's easier and cheaper to buy something that's already done with all the bugs worked out. So, while it may seem expensive to drop $15k-20k on one of these cars, at least you'll have the satisfaction of just getting in and driving as opposed to just buying parts and fixing problems on a cheaper car.
BTW, your post states 87 GN-no t-tops... Does this completely rule out Astro roof cars and 86 model GNs? If not, you may want to state that- could open up a few more possibilities.
There's lots of deals out there but, it may help to be more specific as to what you want/expect.
Good Luck with your quest.
