If you look at Performance / Watt, the server CPUs fare way better than the small cpus.
The small CPUs offer useable power at a lower wattage. But that doesnt mean they have more muscle per Watt. Yes - a 2W Atom delivers way less computations / watthour than a 130W Nehalem
For many parallel tasks the network communication overhead is also often a factor, and you want more power in the same box for this reason.
Look at what the large cluster owners buy. They calculate very clearly on what brings the most bang for the buck.
Therefore: no, a plug computer cluster is not feasible.
In case you want such a system to learn the technology of large cluster computing with many nodes, you will be better off buying one cheapish computer and put a lot of virtual machines on it.