Typical price for the Quest is around £350. I got mine in Maplins.
The mount is the original Garmin unit, so is powered. I have hardwired mine to the battery, but I think I will change this to use the accessory socket as I worry about leaving 'live' pin connections when it is parked.
The mount costs around £40 - £50. I got mine from Rainbow Motorcycles.
The original equipment Garmin cradle fits directly onto the RAM-B-202 circular mount - all 4 holes line up perfectly.
I used a RAM-B-201 3inch arm, and a RAM-B-231 ball joint mounted on the handlebars. Another £50 or so.
Seems waterproof. It has been out in the rain, and splashed through enough Fords and Rivers.