On the Oregon 650 the NiMH Rechargeable Battery 'pack' is in effect two 2.4v 2000mAh batteries with a bit of plastic to hold them together, this bit of plastic presses down a switch in the battery compartment that allows the unit to be used as a charger when plugged into external power, other than that I'm sure that you can get better batteries far cheaper.

EDIT: You may wish to wait for 'Black Friday' to get a good deal online, also last year Garmin themselves issued a 30% discount code purchasing via their own website, maybe they could do this again?