Actually, it's £65.00 for a battery, described as an 'accumulator' on the system, from my local, friendly, BMW Motorrad dealership. You'll forgive me for thinking that's outrageous.
Battery failed on my two year old Nav V, phoned Garmin, gave them the serial number, they said it was still under warranty, sent me one FOC, unplugged the old one, plugged in the new one, worked a treat, no solder, simples.