​Did a preliminary BOM today and selected a few key components, incl. the Spartan 6 variant I'd like to use and the SDRAM and Flash devices (may go with a BGA SP6 package with more IO...no risk yet to the design, regardless of what I choose).  All looks good for ~$25 component cost + PCB (not including Assembly).  Small quantities should still be under $30 assembled and allow me some wiggle room to add other cool things like perhaps a temp sensor or current sensor, what-not.  Alt. is to do a total breakout board as a second shield that gives heaps of IO.  Depends on FPGA choice.  

Also did a preliminary layout (verifying board size and available space).  Even with the QFP FPGA, that is 20x20, the SRAM is 8x8 and the Flash is 6x5.  Awesomely small.  Very cool.  

Next step, power rails and decoupling schema.  Easy enough.  Then build parts and create a schematic.  Stay tuned.