Cubesats and femto-sats are a nice place for universities and small enterprises to set their sights on. There are a number of sources of hardware and software (some even available at reasonably low prices), but outside of specific projects (launch many at once together) I can't seems to find any accurate price model. Data please!
Launching a CubeSat is by their nature a difficult process. They are designed as a secondary payload, which means that the primary payload dictates the entire launch.
There are some programs such as NASA's CubeSat initiative, which provide free rides to educational projects.
For commercial launches, data is spotty. One CubeSat reportedly launched for 100k (OSSI-1). One company charges about 300k for a 3u (Space Flight Services). So 100k per Unit seems like a good ballpark. Both quotes are for LEO.
There have been a few proposals of small rockets designed to just launch CubeSats, but as far as I know none of these have come to fruition.
The bottom line is that CubeSats are by design just catching a ride, so there is no real price model or launching service.
Edit: This answer was written before the question was clarified to specifically address launch costs, and as such does not fully answer the question.
There's a good price breakdown/analysis on SatMagazine for some generic numbers. However, it was written in 2009 so may be a little out of date.
I'm sure there's better data out there, but that's the best I know of. Hope it's helpful.