The KPG-46 is a serial port programmer for those radios, and the
magic behind it is a RS232 to TTL level converter chip that allows the radio to talk to the computer. I built mine on a breadboard and it cost about $11. It was a MAX232, 7 caps and a 78L05. Dirt simple level conversion. I only had to program the radio once (and could rebuild it again if needed) and it worked just fine. The $100+ I see around are ones that have a USB to SERIAL converter built in, and consequently some scumbag markup involved as well.
As the Antenna Farm has a good reputation around these parts, I'd start
here and try theirs for $40 ish.