The answer is ... free!
There was a great demand for these big rocks until about 30 years ago.
However,there is little demand for these rocks today..
That`s why you can get large rocks on the cheap or free in Japan now.
But it cost big money to carry to your site of construction.
We can't move these monsters without a large vehicle and large crane and some assistant.
The holders of the rocks is at a loss what to disposal.
These are obstacle for them now.
I guess there are many people like them in Japan now.
How about in your country?