Personally i've never seen where everyone thinks parts for impreza's are so much more expensive. I think maybe the only difference is there aren't as many used parts available. To lose 2 sec off your quarter mile time for 2-3k in parts doesn't seem ridiculous to me, but maybe i'm not much for comparisons. I think that the boxer motor is probably alot harder to do labor on than say a straight 4 cyl like the evo or srt, but it has its advantages. Like i said in your other post, if you aren't interested in domestics, and you want a new car, i dont' see any problem with making a wrx, evo, or srt a track car. They all have their advantages, and for the money i don't think any one of them is a bad move.
Of course anytime you buy a kit/package for a car with a name brand on it you are gonna pay extra as well. ESX has the fastest daily driven wrx around so buying a setup from them will be steep. I would suggest Gruppe-s.com when looking for parts. They sell evo, wrx, 350z, and s2k parts; so there's your comparsion.

 

If you want a drag car buy Zate's Mustang from him, it's much better suited for dragging...and thenm Zate can buy another Impreza and drive it like it's meant to be driven!