My standard to-go-to FPGA is an Cyclone II EP2C5T144 development board. On a good day, you can find them for less than $5 on eBay. You'll need a USB blaster to

My standard to-go-to FPGA is an Cyclone II EP2C5T144 development board. On a good day, you can find them for less than $5 on eBay. You'll need a USB blaster to program them, which adds another $10 or so. They are small, but they are still large enough to add a little CPU with a little RAM, plenty of hardware multipliers etc.