I at all do not accept the price for a payment with 4 encoders - for example such: http://www.usdigital.com/products/pci4e/
What we in it have? FPGA for $10, PCB for $10 and a small heap of small things on $10. Total $30. And it only a part of that it would be desirable to make for $100 .

There is an idea for the beginning to make the simple scheme with 4 counters, everyone on 32 bits, for connection to a developed drive on ATMega - a drive серво DC the motor. Unfortunately, I am familiar only with AHDL and Quartus from Altera, and for the present never worked with FPGA.

But with the foreign help it is ready to solve this question .