Most affordable FPGA dev kit for learning VHDL and FPGA theory?
One of the Digilent kits would be ideal; I've got one of their Spartan-3 kits. It's an older chip but the board probably has most things on it that you are likely to need, and lots of pins are brought out to connectors for other stuff that you might want to interface to it. They start at $109. Mine has one of the 400k gate chips on it, they don't seem to sell that option any more.

I formed this support group for users. Xilinx has their own forums, as well.