An actual 18 acre lot that’s listed further down is selling for $525,000.
There are a whole community of people living basically ‘off the grid’ although close enough to it to get their social security and disability checks in nearby Niland just East of the Salton Sea in Southern California. Slab City, California - Wikipedia
South of Hilo (Puna, where the soil is not as good) lots can be had for $20,000 for a 3 acre piece of land.
North of Hilo, you’ll usually pay more, but here’s 40 acres for $199,000 (no, you can’t divide it up and sell some of it. The County would never allow it.)
http://www.clarkhawaii.com/property/24546081/32-440-WAILELE-RD-PAPAALOA-HI-96780/MLS-269892
The Big Island is the only island in Hawaii with inexpensive land. This is because it is much larger than the other islands. In fact, it is twice the size of all the other islands combined. The island is about 90 miles long and about 70 miles wide.