The light's design is optimized for use with two AA batteries. With those two AAs it will produce 65 lumens of light for approximately 2.5 hours of run time. With two AAAs it will produce 60 lumens of light for about one hour. With a single CR-123 3V battery it will produce 50 lumens for about three hours.
Now, I consider 60-65 lumens the minimum acceptable for "tactical" work - which includes any use for searching, target identification (at handgun distances), etc. Obviously the AAs pushing 65 lumens for 2.5 hours is the best configuration and performance. For "tactical" work my next choice would be the AAA batteries producing 60 lumens even though it's only for an hour. However, if your use of the light is general utility, then by all means, drop in that CR-123 3V battery and enjoy three hours of 50 lumens. How does that work out for cost per hour of operation?
Around my area CR-123s are sold in pairs for between $11-$13 depending on where you get them. AAs go for about $9 per dozen (I look for them on sale). At $9 per dozen, the AAs are about $0.75 each. The CR-123s are about $5.50 using the lowest sale price I've found. (All of this math goes out the window if you buy your CR-123s online at the usual price of about $20-24 per box of 20). So, one $5.50 3V battery gives you three hours of light. Two $0.75 AAs ($1.50 of batteries) gives you 2.5 hours. $3 of AAs would give you 5 hours. $4.50 of AAs would give you 7.5 hours. I'm sticking with the AAs as my first power option.
A quick Google search revealed some online dealers that have the Gerber Option 60 in stock and being sold for under $40. That's pretty darn good. I encourage you to check out this and other Gerber lights online.
I tip my hat to Gerber for having engineered this.