Usually Macs are a lot more expensive than PCs with the same performance.
But lately they have been improving on this matter.
Virginia tech university in the US built a computer cluster using 1100 G5 macs! That's the power of 1100 computers behaving as one.
They chose mac over PC because they gave them a better deal. Maybe they gave them great prices because it's good publicity for them.
Here are the links:
http://www.chaosmint.com/mac/techclusterphotos/
http://www.tcf.vt.edu/
Doesnt this mac gear look great?
http://www.tcf.vt.edu/systemX.html