$50 gets you a quad-core ARM A9 with 2 GB of DDR3 and 8 GB of flash. Buy twenty. Plug them into a $80 switch with 24 ports.
Hello, kinda slow 80-core cluster with 40 GB RAM and 160 GB flash, plus 80 ARM Mali-400 GPU cores. Total bill of materials $1080.
You get something like peak 192 DP GFLOPS (512 SP) from the CPUs, 480 GFLOPS from the GPUs, 60 GBps aggregate memory bandwidth, 10-20 megs of L2, 5 megs of L1. And all the fun of programming a cluster with a 100 Mbps interconnect.
That said, if you have the right workload, you might have something interesting here. I kinda want to build one just for the fun of it.
Or, wait for the A15 with Mali-T600 series to go mainstream. You'll get double the CPU performance, OpenCL, and triple the GPU perf.