generally, online, high quality brand name CR123A cells (like Energizer and Duracell) go for between $1.17 and $1.25 each with occasional $1.00 each for cells with only a couple of (to a few) years until expiration (this makes them attractive for immediate or near immediate use). these cells have 1500mAh capacity.
one can find, online, "no-name" brand CR123A cells for $0.79 each. even though these cells usually claim 1300mAh capacity or even 1400mAh capacity, they often do NOT perform up to what one might expect. why?...
i've read the Mfr. Spec Sheets on some of these "no-name" cells and the discharge rates are very, very low (lower even than some older single 5mm white LEDs lights would draw). this allows the cell to continue to deliver current for a longer period of time. i won't go into the mechanism/reason involved here, suffice it to say that the capacity of a battery will vary somewhat with the rate of discharge of that battery with higher discharge rates resulting in lower capacity (we'll the theory about this for another post, maybe).
as long as one knows this going in, the "no-name" cells can still be a good value, just don't expect them to last thirteen, or fourteen fifteenths (1300 or 1400 mAh vs. 1500 mAh) as long as a good brand name cell - more like 50% to 75% as long, depending upon the rate at which current is drawn from the cell.
also, remember that these are Li primary cells, and as such partake of the benefits of Li chemistry, e.g., supposed/claimed better resistance to cold temps than alkaline and NiMH cells, less voltage sag than alkalines (due to lower internal resistance), and also a more linear characteristic (i.e. "flatter") discharge curve under loads than alkaline cells.
given the much higher cost per cell of Li AA and Li AAA cells, CR123A cells are a better value. even alkaline AA cells are typically $0.50 each. so, "no-name" CR123A cells are only ~%60 more expensive (though in some applications, this may increase due to the poorer performance of these "no-name" cells esp. in devices placing a greater load on the cell).
IMO, unless one's budget is VERY tight, or many, many cells need to be purchased and used in a short time frame, the added cost is pretty much of a non-issue (sorta' like "majoring in the minors", so to speak, - there are other things that cost much, much more in L/UL backpacking than 20 or so CR123A cells for six mos. or a year's supply - even CR2016 and CR2032 cells for "microlights" cost fifty to eighty cents each and we don't bat an eye at buying them for anemic microlights which supply BRIGHT light for only ~15 minutes and then are at ~25% of their initial starting brightness, and often after 2h use are at ~10% of their initial starting brightness). [Note: i've sworn off microlights; IF i now absolutely NEED a 5mm light; i've gone to a 9V Pak-Lite (personally, i use 9V NiMH batteries as they still last a very long time and they may eventually pay for themselves f i use them enough)].
of course, *AVAILABILITY* (and maybe that's the crux of the matter, otherwise this might be a "no-brainer" of a choice) and availability of good headlamp designs (though there are some good CR123A headlamps available now) is NOT as good as for CR123A cells as for ALKALINE AA or AAA cells (NOT Li AA/AAA cells - at least in stores where i've shopped recently - in fact, i've found, in brick-and-mortar stores, very expensive Duracell CR123A cells when there were no Li AA/AAA cells were even stocked by the store) .
lastly, when used in modern "regulated" headlamps and flashlights, one can consider in practical use that the stored energy is roughly equivalent b/t CR123A and both alk. and Li AA cells with CR123A cells coming out just a tad ahead of Li AA and even a bit more ahead of alk. AA cells. remember, the CR123A cell is delivering its energy at a higher voltage than AA cells (roughly 2x the voltage of both alk. and Li AA/AAA cells). this fact allows the boost converters and regulator electronics in some modern (properly designed) lighting devices to draw less current from the cell to supply the proper current to the load. nominal voltage on CR123A cells is usually ~+3.25VDC to ~+3.35VDC on "fresh" cells that i've measured on a DMM.
just some thoughts.
my two shekels, YMMV. ok. 'nuff said.