Using a Raspberry Pi is definitely a space/cost saver. Compared to a PC, its a fraction of the size, costs $25/$35 for 256MB/512MB models respectively, and uses much lower amount of power (5V micro-USB charger). Things like heatsinks and fans add a little bit of cost, but definitely improve the performance (I know this from experience).

If you have an old PC layout around, you could certainly use it. Over time the extra energy costs will definitely add up though.