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I bought this one for $2.50 ($2.99 now) and it works great on my garmin.
MOTORCYCLE/BIKE HANDLEBAR MOUNT HOLDER GARMIN NUVI GPS - eBay (item 250743582829 end time Jan-14-11 23:04:39 PST)
the quality seems to be just as solid as my window mount that came with the garmin. you just clip on the bracket from your windshield mount that came with your gps to the ball mount and your done.
pic of it installed:
and if I need a waterproof gps I simply stick a 2 cent ziplock bag over it. It is a lot cheaper than buying an expensive motorcycle specific gps. if you are really concerned you can get a cheap garmin (best maps in my opinion) and buy the extra accidental damage insurance through squaretrade (about $20) and then if you drop it, wreck or forget your ziplock bag and it gets wet it is still covered and you will get the original purchase price back. squaretrade warranties kick butt and they send you 40% off coupons all the time for warranties.