The sellers are going to have a hard time getting their money out of that. I imagine "high 6 figures" is right. Even 7 figures. It's a vanity house, I suspect, built by people who found some success and decided to build a house in a cave.
It seems, from the pics, they did a great job and I assume it's pretty liveable. But it's still the owner's dream, not mine.
I've seen a lot of "dream homes" built by financially successful folks sell at auction and bring a fraction of the construction cost.
If you are willing and able to spend in the highest brackets for a home, wouldn't you want your own guitar-shaped swimming pool?