I have a cheapo Nuvi52LM that I kept in my car as a backup because I spend a LOT of time in the mountains and out of cell range(so google maps doesn't work). I took that gps and bought a $18 GPS case/mount for the bike, specifically this: http://www.amazon.com/ChargerCity-Resistant-Bicycle-Motorcycle-Security/dp/B00JAULSTI?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage
It works great but it's kind of tough in direct sunlight due to the lack of a shade but for $18 (or $18+cost of gps) you really can't beat it. I wanted a moto gps but I have a very hard time dropping 4 or 5x as much on a product just because it's a "niche" market.
I use a Nuvi55. The secret is a $20 water resistant case. I use it on my snow-mobile no problem. Here is link for the one I use (I only use the case, not the mount) https://www.amazon.com/gp/product/B00JAULSTI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1