Consider: We live on a ball of molten iron. Dig a deep enough hole and you get a nearly unlimited supply of thermal energy.
Early books (1860s to 1880s) on oil, gas, and coal noteed that tthe United States' coal reserves were good for one millon years, _at then-present rates of consumption._
The rates increased. A few centuries, tops, now.
British coal extraction peaked in the early 20th century.
UCSD physicist Tom Murphy extrapolates human acttivitty to galactic-sscale energy conssumption in fairly short time.
http://physics.ucsd.edu/do-the-math/2011/07/galactic-scale-e...
In that way it's very similar to oil.. no one has to pay to create the energy contained in oil, but they do have to pay to dig it up and distribute it. And yet, oil is not even close to being free.
Edit: a better comparison would probably be hydro.. which also isnt free.
Iceland uses geothermal. They have it easy due to geological circumstances, but that's mostly a cost-saving measure. Any country can do the same.
There's also geothermal systems for homes that cost far less that work by tapping the temperature differential in the ground: https://www.thisoldhouse.com/ideas/geothermal-heat-pump-how-...
Oil is expensive because it's a scarce resource that requires a lot of effort to extract, process and transport.
A geothermal system for your house involves "transporting energy to your home" but the distances involved are so minimal that once installed it's next to nothing.
> Since placing your home next to a large temperature differential is difficult...
Well, that's wrong. Every house sits on top of a significant temperature differential which is why geothermal systems work. In large parts of the continental US the ground deep beneath the house maintains a consistent temperature year round. This can be used as a thermal sink in the summer, and a source of heat in the winter.
That being said, until we have highly optimized how we utilize that unlimited source, it will remain a scarce resource.