How handy are you with soldering irons?
I wasn't gonna waste $40+shipping for a $2 cable, plus I didn't want to wait for a week, so I ended up making my own cable with a serial plug and a stereo jack for my Garmin Foretrex. I got the pin configuration from this site:
http://pinouts.ru/DevicesCables/garmin_foretrex_pinout.shtml
Worked like a charm. I had a 9-pin serial plug and a stereo plug both came from a box of surplus cables I kept over the years. In the end, it took me half an hour to put it together, just soldering the right connections together.
As for the USB part, I'm not sure that the software you run (GSAK?) that would communicate with your Garmin would recognize your GPSr as being connected to your USB port (it would look for a COM (serial) port, not a USB). But I leave that one to the experts. I know GSAK can do many wonders, after all.