I find it interesting that you say "Europe" in this context. As someone who travels around Europe a fair amount, I've researched mobile phone tariffs in various countries, and the differences can be staggering - for example, an order of magnitude difference for the cheapest prepaid data tariff seems pretty common, calls and SMS vary wildly as well. I haven't analysed it in depth, but there aren't any obvious correlations with population density, size, GDP/capita, etc. as you might expect. Not even the number of network operators (=competition) seems relevant.