I don't think there is a definitive answer. If it was scarcity or supply only, the Spirit of America bikes built in the Summer of 1978 would be the most valuable as there were only 200. The 1976 900B LTD was also actually "limited production" but does not command the same prices as it'd Z1 predecessors. The 1973 models value is easy to understand as has been said, because of it's groundbreaking performance and being the first of many more refined Z bikes to come. The original was a little rough around the edges, which I believe is part of the appeal, also the amazing graphics/color scheme with the Rootbeer and Orange which IMHO has never been surpassed! I was just reading a period magazine test that conjectured that the 73 engines were painted flat black so people could easily recognize that they weren't looking at a Honda CB750, Ha Ha!
Mike
Former M.E. at Kawasaki Motors Manufacturing, Lincoln, NE
1966 W1 (the Z1 of 1966-50H.P. and 100mph!)
1974 Z1
1978 KZ1000 LTD
1976 KZ900B pile O parts
1980 KZ750E
1980 Honda XL250S (I know, wrong flavor!)