I'm a CPF regular and have lost interest in 123-powered lights. The Fenix L1p is my favorite now, 1AA powered (usually) by an AA NiMH cell. The new Sanyo Eneloop and Amondo/Titanium Enduro cells more or less solve the NiMH shelf life problem except in cold weather. And there's always L91 lithium AA cells (unfortunately Batterystation discontinued their alternative). As for cost per unit energy:

Batterystation CR123A, 3.0 volts, 1400 mAH, $1.00 = $.24/WH
Energizer L91, 1.7 volts 2900 mAH, $1.75 = $.35/WH not that much worse than 123's
Sanyo Eneloop AA, 1.25 volts 2000 mAH, $3.00 = $1.20/WH (but rechargeable)
Sanyo HR-3U 2700 mAH 1.25V $3.00 = $.8/WH (limited shelf life, rechargeable)
Generic NiMH AA, 1.25V 2000 mAH $1.00 = $.40/wh (limited shelf life, rechargeable)
Generic alkaline AA = 1.3 volts (average over runtime) 2500 mAH, $0.25 = $.08/WH (reasonable shelf life, not suitable for high powered lights)

In practice I use NiMH cells in practically everything, with $/WH close to zero since I recharge them. I'm using mostly Sanyo 2500 mAH HR-3U ($2.50 each) but am probably going to slowly migrate to Eneloops for most purposes. I do have a bunch of alkalines and lithium AA's around just in case. And I can charge the NiMH cells from 12 volts in my car, if the regular power goes out. But I usually have some charged NiMH in a camera or something that I can scrounge from.

Finally, don't forget larger cells like D cells, still very useful and cost effective for some purposes.