I basically copied the Ganglion design, but removed the SD card holder, accelerometer, and the obsolete bluetooth/microcontroller module. I also hard wired all the instrumentation inverting inputs to the reference voltage since I only plan to do single-ended sensing. I was able to squeeze everything in to a pretty tight 2-layer board. I haven't worked out the math yet but at ~$2 per board (including shipping), 4 instrumentation amp ICs and a few other $2-$3 ICs, and the main AFE chip (MCP3912) the whole thing comes out to be probably $20-$30 bucks, so easily in most people's budgets.