Here's another person living in a 727: https://www.youtube.com/watch/?v=rKm5oF2p-II
This one has a much nicer interior: https://www.youtube.com/watch?v=bdwLlI9abgU
According to one of the comments on the first video, they spent 120k on the plane, and another 100k to move it to the forest. I would probably want to spend another ~300k for renovations and everything else (proper foundation, shower, kitchen, water and sewer lines, etc. etc.)
It's not cheap, but that's a lot cheaper than I was expecting. Especially compared to the average house in an expensive city. I think it might also be a lot more fun than living in a "tiny house".
I recently read this "Why We Don't Like Our Underground House" article that was posted on HN: https://dengarden.com/misc/The-Pitfalls-of-an-Underground-Ho... That was a reminder that doing something unconventional can be risky and cause lots of unexpected problems. So it would probably be a good idea to also spend a lot of money on architects and engineers to make sure that everything is done properly and there's no surprises.