IBM's System z running Linux seems to be cheaper to run as compared against a farm of servers, each running specific applications. Illuminata analyzed data provided by IBM to conclude that "IBM has demonstrated that mainframe total cost of ownership (TCO) advantages are 30% to 60% better than either 30 Sun Solaris servers or 300 linux servers." More detailed info can be found at Computerworld.
If you want to test it yourself but cannot afford to buy the IBM mainframe, a friend of mine, Ms. Arlene Gagno-de Manuel, sent me information on IBM's program to that gives you free accounts and full access to Linux running on System z.
I wish I had the time to test drive it for you but as it is, I am swamped with work from my day job. :) [yeah, I know - "excuses! excuses!"]
[image courtesy of IBM]