What's the expensive part when you've got free heat?

Literally nothing. But the heat is never actually free. You have to build and run a bunch of infrastructure to turn the raw heat into steam. Even if you're iceland with "free" geothermal you need to spend money to capture the heat as steam.

If steam were free like the wind nobody would care.

Comparing efficiency is a really apples to oranges way to go about this and should be a massive red flag that someone has an axe to grind.

Comparisons across different forms of generation typically look at cost (in dollars, carbon output or any other metric you want) per energy over the life of the thing doing the generating or per some multi year period.

Yeah, that's why I was asking. Why do we even care about efficiency? All I care about is dollars per watt/hour.

The last coal plant built in the US cost $600M for 900MW. Most of the cost of a coal plant is the apparatus for turning heat into electricity (the turbine etc).

Even if you have free heat, turning that free heat into electricity is more expensive than solar electricity.

That's power, you want energy. What is the total cost in dollars per Wh?

The entire part where you need to build machines to gather, use and cool it down.