I've looked around, and didn't like the prices either. $29 +/- for a USB dual and handle bar mount. Found this at Princess Auto (Harbor Freight in the US), it was on sale for $3.99, and I mounted one on my GW in a pouch.

I own 2 more for mounting in other places, like the trunk, so I can power other things with a USB cable, while riding. Also, I wired them hot. I don't think I would ever leave my GPS or cell on the bike over night, therefore no relay.
Just remembered, here is my next project.

I bought a cheap Hong Kong temp gauge, and 3 of these miniature variable power supplies. My intention is to take the 12v bike power, and hard wire in my garage remote, a Blue Tooth for my cell connected to the intercom, and the temp gauge, all on a relay. These power supplies where like $3.10 ea, and I figure I'll water proof them by imbedding them in caulking.