nevermind, there are comprehensive posts on the sort-of-duplicate thread
Nordic makes an awesome set of chips that have developed a community following.
I like in particular the NRF24L01+ 2.4ghz transceiver chips, available in convenient breakout+antennae form for cheap.
(3.20USD off ebay, ~20 off of sparkfun and that plugs right into sparkfun's awesome nordic serial inteface board (which is another $20), ~5 to 7 on non-ebay sites iTead, MDFLY, etc. -- the ebay and non-sparkfun are identical boards being mass-manufactured somewhere in China it seems). There's arduino and maple libraries written for them, in addition to the sparkfun code, and diyembedded has tutorials. We've used both sparkfun and mdfly boards with success.
For even more plug n play check out jeenode. Not sure how low-power it is.
Bare chips run 1.80 on semiconductorstore or 3.50 on sparkfun from quantities of one (SS has more exp. shipping than SFE though, and not sure speed). Nordic also makes SoC versions with a mcu -- haven't played with those yet.
If you want to talk to smartphones, nordic also makes cute ULP bluetooth chips nRF8001, but they are bluetooth 4.0 or something and no one in the maker community seems to have used them yet...
comparison of nordic offerings: previouswww.nordicsemi .com/index.cfm?obj=menu&act=displayMenu&men=21 and mouser .com/catalog/catalogUSD/644/2773.pdf
I also searched for theses. There is a good paper with cost/power/range comparison, but it's from 2009, "A Comparative Review of Wireless Sensor Network Mote Technologies" and doesn't talk about nordic at all, so it may be a bit out of date
see also community here
Good luck! This area is new to me and I look forward to other people's answers.