alt Hacker NewsUNIX was only cheap because AT&T was forbidden to profit from it, had it not been the case, it would have been a proprietary closed source OS, exactly at similar price points.
The proof being the graphical UNIX workstations from Cray, SGI, Sun, NeXT that came later into the market.
The dimension you're missing here is that most of the Lisp OSs were single-user (AFAIK initially all of them were but some gained multi-user support later) in a period when timeshare was king. Not to mention that the hardware itself cost multiple times more to manufacture.