if i remember correctly the fronts are vented and rears are not on a stock RS, the rears are a bit more prone to warp, but it depends on how hard/ often your stopping, it may be incorrdct info, but i have always heard that hard stopping, followed by a lengthy stop where the pads are held to the rotor will cause warping to occur.
as for impact on the bank account.. i look at pads and rotors as a wear item, so cheap stock blank rotors for $15-$20 that may only last 1-2 races is a better choice for my wallet, than hoping to get 20 races from a $300 rotor.
http://www.partsgeek.com/catalog/1998/subaru/impreza/brake/brake_rotor.html
I be sorry as a muthafucka I did, still sorry I did n' hustled ta peep what tha fuck I holla'd a lil' bit better, or at least try to.