what is the cheapest modification that can be done to increase torque on a 2006 Subaru Impreza WRX

replace the old drive shaft with a lightweight one. Personally l would save up to get a turbocharger.

The entire exhaust system frees up the most horsepower. Oh wait, cheapest, most likely an intake system.

Neither upgrading the drive shaft or changing the intake will produce any more hp or torque for ur subi. The intake will do nothing but suck in a bunch of hot air that ur radiator produces, thus warming up the stock intercooler & on hot days u will experience power loss & in some cases bogging. The best & cheapest way to produce power is either w/ an accessport which will allow u to run a more smoother map, wo not give u much but will broaden ur power curve and/or upgrading ur downpipe which will free up more air to pass through.