Think about it this way: in the days before motorised transport, were prisoners always kept within easy travelling distance of their relatives? I think not, and it wasn't cruel-and-unusual then.

For practical purposes, though, Montana to California is a bit too far. But Nevada spends only $20K, and there's not much of Calfornia which isn't within a reasonable daytrip of somewhere in Nevada.

Mexico probably spends even less, though contracting prisons out to Mexico might cause genuine constitutional issues...