Do you want just a disposable driver or are your intentions to restore it?
I would pay no more than $4000CDN ($3000USD)(if it had the original drivetrain) and budget for another $3 to $4K on top of that to do it right. If you can see 'just a little' rust, it's the rust you can't see that will cause you($$ and time) problems. Are you capable of doing your own work?
Do the usual mechanical items check that usually are cheap and easy to fix - brakes, brake and fuel lines, steering linkages etc
The following is what may drive repair/restore costs through the ceiling>>>>
-check for any repainted areas that may hide poor body repairs
-from under the pass side floor check for holes(4 screws, remove door sill plate and lift carpet)
-same as above for driver's side
-look in the trunk..can you see the outside anywhere?
-inspect/hammer the frame 1 foot to the rear of the back tires - look for holes or jagged pieces of frame
-bounce the car up and down by the rear bumper - does the bumper flex?
-push on the rockers behind the rear wheels
-open the doors partially and lift and push with force and note looseness
-inspect door hinge/cowl area for rust
-carefully open and close anything that opens and note any noise or higher effort to close
Does your area have emission regs, i.e.: Does OEM emission equipment need to be installed to be legal?
FYI: I bought a 90,000 mile, 5 previous owner, excellent condition, partial repaint, all OEM 86 SS last year for $10K CDN($7500 USD)
-so far I've replaced body mount bushings, brake flex lines, water pump, door hinges, replaced all fluids, performed lots of minor repairs and maintenance to the body and mechanical assemblies, very minor rust abatement < this was a car that appeared ' mint ' looking.