Steve, I have had my XK for well over 3 years now, and all it has cost me is one indicator bulb and one brake light bulb, about ten quid total.
OK, the Road Fund Licence is 500 quid, and you may be right about MPG, but I never check that: I only do about 3000 miles a year, so the cost of petrol is not a major concern.
I simply love my XK, and intend to keep it until it is too low for me to get in and out unaided!