Momentarily off topic, but this doesn't sound right, mate.
I get at least double that, and use the green/red Uniross 2100mAh NiMH Low self discharge ones here:
http://www.batterylo...ble-battery.htm
The principle thing for me is the low self discharge bit. I use the GPS fairly rarely (the FNRttC, and maybe the odd social in between) and suspect I lose more charge through chemistry than drain. A pair of 2100s will normally last me for a good 26 hours riding, spread over about a month. That said, Adam loaned me some 2700s on a recent FNRttC, and I put about 450 miles on them before they started flagging 'low'.
I bought the eTrex because battery technology just gets better and cheaper, whilst an individual (proprietary) battery gets simultaneously worse at holding a charge and harder to replace. AAs for me, all the way.
For a real shocker, try whatever variety of Energizer is currently packaged with the greatest number of superlatives and lightning strikes. IIRC, I changed the batteries once during LEL, and that was 100% routing with intermittent backlight.