I was quite surprised that the University of Florida wasn't mentioned, they have several online doctoral programs.
Montana State U. (It's in Bozeman, near Yellowstone, while U. Montana is in Missoula, off to the west.) I would guess that Montana State sits atop the list because its a reasonably good school whose annual tuition is only $5,712. That's second lowest of the 25 schools listed. So the authors probably thought that it qualifies as best-buy.
Yes they do. But from one of their Ph.D. programs: "The current cost, which has remained unchanged for a couple of years, is $544.56 per credit hour." That would take them above $15,000 per academic year, which (the article says) is the cutoff point for the list. J.