How much does one of them go for?

You can get them for about 2000 euros in Europe. Variation in prices are mostly due to shipping from China.

Some time ago I was looking for a chassis like that one, minus the lawnmower hardware, to build something that could "patrol" around my home, which has some irregular terrain that would probably have anything with non huge wheels flip over or get stuck. All I could find are interesting metal robot chassis on Aliexpress which were however way overpriced considering the much smaller size and crappy plastic tracks.