You can get a great cheap ITX cases these days for about $50 (Cougar), A 650w SFX Power Supply for about $70 (Evga), ITX motherboards start at $110... then just make sure you choose a sensible CPU/GPU from there based on your power supply. And if you're using entry level cpu/gpu then you don't need to go crazy with cooling either.

Certainly not much more expensive than a regular mATX build imho.