I`ve always thought that bike sat navs are a stupid price when you can buy a car one for around £50, all you would need is a waterproof case that would fit on the bike so I recently bought a GPS holder for my trip to france this year.
Xtream Wearther SatNav Pouch
It worked well but I`ve recently replaced it with a tank bag which has a clear pocket for a GPS from Kappa for £39.99.
Kappa link
It`s really good and has lots of space to put other stuff, it`s also fairly easy to use the touch screen on the GPS with summer gloves. As it sits fairly tall on the tank it`s quite easy to see.
The GPS I use is a Navigo, £49.99 which worked great on my trip to the South of France this year.
Navigo for sale at WH Smith`s
The software that comes with it isn`t brilliant but it can be "re-configured" as shown on this site:
Navigo adjustment site