It's about $3 at Walmart (in store, not listed at web site). There is a similar one with a pushbutton switch -- a penlight style -- but the other is a twisty, although you can fudge it a bit by almost tightening it enough and then using the play to turn it on momentarily.
Both use a single AAA but neither of these are very bright, although I always have one in my pocket. There are better lights available, but these are pretty good for the price.
The ITP A3 eos (a twisty switch) and the Maratec lights (both about $20) can go much brighter -- use the google search function at the top of this page to find reviews on them.
Dorcy (look at
http://www.dorcydirect.com/c-1-led-flashlights.aspx ) also has nice little keychain lights, only about $6 or $8 but not very bright such as 41-4224 1AAA, among their offerings.
Fenix also has good lights in this class (look at 4sevens.com).
Someone may mention Photon lights, but I don't have any so I can't tell you about them. Check out 4seven's new Preons too.
Yeah -- there's a LOT of nice little lights around, but the Ray-o-vacs at Walmart are probably the cheapest if all you want is a sturdy light with low level to see something a few feet away. But these are like peanuts -- nobody can eat/buy just one.