you can easily get an s4 now for 20k i got mine for 16k with 44 thousand miles, and you would probably spend 15k on a used wrx, but the s4 is faster, alot nicer, the sterio is like 50 times better, and the quattro system is just plain better.
i was trying to decide on either getting a wrx or an s4 and i drove like 5 wrx's and 3 s4's and each wrx i tried had a problem, on two of them the cupholder was broken, one vibrated like crazy, and one the cupholder, the ac vents, and the shift knob were messed up. while the only problem with the s4's i found was that on one the thing you pull to open the hood broke so audi may cost more but their not overpriced their just better.
and audi has won alot more rallys with their quattro system than subaru, audi pretty much invented awd, and their made by germans, while subaru is made by japanese.
im not saying subaru's are bad cars they are really cool especially the sti, but compared to audi they just arent as good.