I dunno... I think that Rallying a BONE STOCK 1999-2000 Impreza 2.5RS would be pretty cheap.

I ran a year of rallying with a 01 RS motor (yes it had some cams), and certainly didn't cost very much.    

I ran the following:
- "street performance" brake pads, the one set last one season, and still had LOTS of meat left over
- Pump Gas (91 because i felt bad putting 87 in a race car)
- Rotella T oil ($20 oil change before every event)
- The entire season on 6 tires. (not including snows etc for pines)

I broke one driveshaft...but it was a 200,000+ Km unit.  

Talk to Randy Zimmer....he runs his car for next to nothing.

Once you get into modifying it...then yes some things get more expensive.   Things like Diffs (because there are 3 of them).... but most of the other stuff is about the same....Brakes, Power, etc.


Having said that.  I think a Focus, or a Honda Civic would be really cheap to run, and fun.