Personally, I think for "rural" areas the cheapest would be satellite. As DSL is largly distance limited, I don't envision repeaters being installed for rural areas. I don't think there would be much return on the investment, unless we maybe have differing definitions of what "rural" is. Also, if by "cheapest to impliment" you mean by the end user, than really i think satellite is your only choice anyhow. So really, I think xDSL would be towards the bottom of that list, mainly because repeaters are not likely to be installed so a couple people can access the net via DSL.
Of course, I could be wrong...
\Dan