I went around the same dilemma myself and couldn't justify the price of the waterproof satnav. After a bit of messing about this is what I did:

Wired a USB 12v charger to my battery.
Bought a basic holder off fleabay for about �20. It's pretty good even in torrential rain but it ain't water tight.
Downloaded navfree for road riding and memorymap for trails to my android phone. Total was around �10 including some OS map downloads.

All in, it was under �50 and it keeps my phone charged while I'm out and about...

Although, I am presently messing about with it. Siliconed up a Tupperware box so that I can charge a gopro out and about and swopping the basic case for a modified drybag to keep the phone water tight. Happy to post some pics and links if anyone is interested.