Another option is to buy 223 batteries. 223 batteries are nothing more than two 123 cells connected in series. All you need to do is cut open the plastic housing and voila, two 123's. Last time I looked,
www.cheapbatteries.com had duracell 223's at $1/ea for 100+. 100 223's= 200 123's, so that's less than $0.50/ea.
Now, the downside is that is it a bit more work. How much depends on the style of 223 you get. I've seen two - one style has a casing that has open slots on the side. These are easiest to break open, and the cells are already insulated. The other style is completely enclosed. These are a bit more work to get open and the cells are uninsulated. You'll need to wrap them in electrical tape or use some heat shrink wrap. If you shrink wrap them, you can just shrink wrap 2 or three cells together if you are using a 6v or 9v light.
Rocko