I used to do a lot [more] HPC software development when I had access to the Penguin cluster at a previous place of employment. The cluster was a full upright rack full of nodes.
As I get more into node.js (and some other stuff: you can do MPI in python), I decided to build a small cluster for development purposes. After my really good initial experiences with a pcduino3, I decided to use the pcduino3 nano instead of raspberry pi’s. The pcduino3 has a lot more power than the rpi and you can find it for about the same price if you shop sales (checkout LinkSprite).
With the proliferation of cheap gigabit switches and high amperage multi-port USB chargers (like the EZOPower 96 watt 8-Port) you can build a fairly affordable low end cluster that is not very big.