One of the items I've been meaning to add to my wireless tutorial is a discussion of actual coverage areas for different frequency bands. This matters because, with today's mobile radio technology, lower frequencies cover more distance and do better at penetrating buildings. That means fewer cell sites for equivalent coverage and thus lower infrastructure costs.
Indeed, a little discussed issue in the US is that Verizon and AT&T own the 850 MHz cellular spectrum and they were the big winners in the recent 700 MHz auctions. Sprint PCS and T-Mobile USA use spectrum at 1900 MHz or 1700 MHz, so they need more cell sites to achieve similar coverage. They are at a cost disadvantage.
Signals go even further using the 450 MHz spectrum that's available in some countries. I've argued publicly that the best thing developing countries can do to bring mobile coverage to rural areas (for example in India), or to remote areas in general, is to make 450 MHz spectrum available to mobile operators.
To get a sense of how significant these effects are, here's a table that Qualcomm submitted to the ITU's Working Party 8F several years ago. (Thanks to Joe Nordgaard for the pointer).
Source: Qualcomm ITU 8/F Submission, June 11, 2001, “Coverage comparison of systems at various frequency ranges, including 450 MHz”