CPU mining is bad; they're not that well suited for hashing as GPUs, their cost is too much in comparison to their hashing speeds and you can't scale them (1 PC = 1 CPU, maybe 2).
Yes CPUs are just too expensive, and require lots of space (1mobo per CPU). or if you get one of those server mobo, which are like >600$ you might get 4 on one board.
Personally i wouldnt do it
Besides from it is expensive. The hashrate and the duration of usage of cpu is too short compared to GPU. I already try that using the workstation computer in my office. Intel xeon cpu. But it only gave me a penny for almost a day of mining using minergate app.