The cheapest converter I've found is about $150 but you might be able to get one for less at Ebay. You have to balance that cost against the time, hassle, and cost of doing it yourself.
If you use the PIC converter, than you should be able to program the PIC to talk directly to the GPIB bus without needing the USB port to talk to another microcontroller, which just adds an unnecessary interface. I assume you could interface to the PIC to tell it what commands you need sent by some type of keypad.