Wish there is an affordable ARM SOC board for building DIY Spark clusters, which has a 64bit CPU, 8 cores, gigabit ethernet, SATA, USB3, and 4GB memory (8GB memory would be even better), and under $100.
Spec-wise, Odroid xu4 [1] from Hardkernel is very close to meet this requirement, though still lacking in cpu and memory(only 32bit and 2GB memory for xu4).
I would use it for personal projects at home. Just for fun.
Though I would envision there might be a niche market for affordable Spark clusters as appliances, say, one 1U box that contains 20 boards with total 160cores, 160GB memory, etc and only 200watts power consumption.
Spec-wise, Odroid xu4 [1] from Hardkernel is very close to meet this requirement, though still lacking in cpu and memory(only 32bit and 2GB memory for xu4).
[1] http://www.hardkernel.com/main/products/prdt_info.php