Cost comparisons of using server class machines and commodity machines have been worked out in the paper:
https://cs.uwaterloo.ca/research/tr/2009/CS-2009-09.pdf
Basically for google-like usecase we calculate how many hard disks and compute power we need.
and we calculate the cost with both for server class and commodity machines.
STEP 1:
Calculating the size of the web and the size of search index, and estimate how many hard disks we need :
STEP 2:
Compare the costs for CPU and disks with server class machine and commodity machine:
