roughly $800 to about $1300 for an assembled pair. Worth it if you have the money though.
Junkyard stuff...you'lll probably get what you pay for. It's doubtful you'll find a pair of real nice heads at a junkyard that haven't already gotten taken.
If your engine is still a stock c code, imo, you would notice a significant improvement if you swap out the intake and cam. the c code cams are really weak, you can get a mild cam that will keep a nice idle and give a significant improvement in horsepower. a carb and intake will run you somewhere in the $400 range and is easy to install. a cam and lifters will run somewhere in the $150 range and is significantly more difficult to install. it involves either dissasembling most of the front end of the car, or pulling the engine. hope this helps