I guess you have to think about including the cost of a PC in there though. If you were just playing M:TW and R:TW on your PC that you bought for £800 then your figures would look different:

Total hours in Total War : 500 + 150 = 650
Total cost of Total War: £25 + £40 + £800 = £865

Cost per hour: £1.33 per hour.

Still very low I think compared to many things.

Have you really played 650 hours of Total War though? Thats nearly a solid month of Total War?
I am impressed and a little scared.