Does it have to be Garmin? If so, I've found bunches on Ebay.
AnywhereMap has a new device they JUST put out that looks good. Cheaper than either one, and upgradeable.
This isn't a PDA-based unit - it's a dedicated, special build.
http://anywheremap.com/anywheremap-atc.aspx
Leaning heavily toward this one right now.