The latest Pi looks to be a box I can wrap my ARMs around. pun intended. About 15 years ago I built a cluster of Linux servers that had fanless Intel CPUs that clocked in a around 900Mhz. Maybe that’s the sweetspot or material dynamics. If I recall each system cost me about 350-500 to build. The motherboard was probably around $125 and the bulk of the expense was the case, power supply, cdrom, and hard drive. With the new Pi and the state of art injection molding and new plastics I could build the same cluster for a fraction of the price.
One negative attribute is the RAM. The Pi-2 only comes with 1GB and while that might be ok for a small OS like CoreOS, busybox or scratch I don’t think it’ll handle many containers or something that can act as a POC. However a cost analysis should be performed because maybe an adhoc Google Compute Engine cluster would make more sense.