I think you can find a freestanding house in former DDR for less than 300k but it will certainly not come with a pool! Maybe a mud puddle if you are lucky.

> You can find better deals in the US, but probably none with tunnels

I once came across a middle-of-nowhere midwestern house auction listing by the US government. It came with a detached 5-vehicle garage, and an attached multistory underground hole I assumed was a missile silo. I can't remember the exact starting price, but it was significantly lower than $400k IIRC.