I've used a
Garmin GPSMAP 76CSx for the last couple of years. It's waterproof, easy to use, and you can find them new on eBay for under $250.
Mine is mounted using the plastic
Garmin Rail Bracket, reinforced with a couple of very large o-rings (to eliminate vibration).
I have since moved the GPS to the left side of my handlebars.
It has been mounted like this for nearly every one of the 20,000 miles that I've racked up.
