[inquisitor]

Of course the relative costs of building and operating a network increase with the population density, but a far more relevant factor is competition. The more operators compete in a country, the lower are the tariffs (as long as there are no cartels like in Eastern European countries).
Austria isn't a good example, because they have a lot of tourism and so operators have huge earnings from roaming customers. Before EU-regulation of roaming tariffs became effective roaming customers accounted for approx. 30% of Austrian operators' revenue.