It's a question of what you compare it. A fully built N-spec rally car(Mitsubishi Evo, Subaru STI etc.etc) can easily cost between 150K$ to 200K$.
These M3's are usually driven by hobby rally drivers, with rather low budjet. You can buy a decent M3 from Germany between 10K-15K€ and spend about another 15K€ to build it, so with 30K€ you get a low budjet rally car. Earlier there were a lot of old Ford Escorts (european mark II), but they have been replaced by "newer" cars, like M3's. It's rather difficult to find nowadays rear wheel drive car from which to start build a rally car.