Depends on if you do the work for one and which 5spd gears you go with. Dont forget small things like if you have a short shifter on your 5spd, you wont be able to use it with your 6spd. I just did a 5spd build on my car, ran me around $6500 (1-5 gears, front lsd, clutch, flywheel, fluids and labor). Im not sure what 6 spds would go (Im sure just about any of them). But you need more parts (i.e usually hubs, different driveshaft and other parts).
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX