> Terrestrial cellular towers cost between $150k to $500k per tower

I'd be interested to find out exactly where this cost exists. I would expect the majority of the cost (especially in rural/mountainous areas) to be more with power and backhaul, rather than the physical radio gear. Because it's rural, you should be able to easily just use coverage bands (ie 850 MHz or 900 MHz) with relatively high transmission power. This would easily be able to cover 300 km2.

Because of the higher transmission power, and the fact that the tower would be in the middle of nowhere, wouldn't the OPEX be higher, with smaller numbers for CAPEX?

A lot of the cost is regulatory. I used to work at a mobile provider, and it took months to get permission from all the various government agencies before we could actually start building. Even if the tower is in BFE, you still have to get all your plots to the FCC, you need EPA signoff for batteries and fuel tanks and such. Plus there's always state and local permits of various kinds. We had a custom workflow application just to track all of that and there were dozens of steps.

Cell towers aren't very expensive on an ongoing basis, but every few years you're rolling out the next big technology (we went from analog to 1x to 3g to LTE while I was there) and it's a headache.