I belong to Hagerty Insurance and use their price guide often. They constantly update it using auction house numbers etc.
These are of course LHD US cars. #1 cars are in the museum, #2 are excellent examples, #3 are good looking driver cars, and #4 are needing work like paint, chrome, interior but otherwise run and drive.
I will list some #2, #3, #4 cars because that is what we drive and aspire to.
1970 Zed- #2 $34,600....#3 $16,000.....#4 $8000.....I think #3 is closer to $20,000 and #4 closer to $12,000
1972 Zed- #2 $33,400....#3 $16,000...and #4 $7200....all pretty close I think.
1973 Zed- #2 $33,400....#3 $16,000...and #4 $7200....maybe a little high as the 72's bring a bit more
1974 Zed (260) #2 $15,100...#3 $7800....#4 $4500....low by %40 to %50 I think. I would buy #2 cars all day long at $15,000.