The Hemmings Sports & Exotic Car review article in 9/2011 of the 1974 CS with 4,500 miles on it indicated the owner (son of a BMW dealer in the '70's) stated the 'book price' in '74 was $10-12K for CS's, but buyers actually paid $15-16K in that era. So, 50-90% escalation between '70 and '74.