That advert is quite informative and at $389 in 1975, allowing for the change in the value of the ÂŁ to $ in the last 42 years and the rate of inflation in UK over those years, means the price in UK at today's prices would be around ÂŁ2500. (thank goodness for information about the values of money on the web). It would not have been a cheap camera even today! It would have cost more than my Nikon F6.
That said it is quite a quirky camera and takes some getting used to but once that is sorted it is a delight to use and very quiet when the shutter fires. (Less than my F6).
As far as I can tell the main difference between the XK/XM was the auto exposure facility. The flash sychro speed was 1/100th of a second but the slowest manual speed on the F2 was if I remember correctly 16 seconds. Oh yes there was no motor drive except on the ones with the fixed motor. I certainly don't need one of those - landscapes don't often move very quickly