I like to track the A66 Challengers and have owed a Y-1 convertible, automatic car since 1979.
My own tracking of these cars seems to be in the following ranges:
A66 Convertibles in restored/original condition seem to sell in the 60-70k range. This is the top price I have seen for convertibles in stock, collectible condition by dealers and at the auctions.
Hardtop A66 cars seem to sell in the 40-60k tops in similar condition. Clean drivers in various conditions sell in the 27-40K range.
Hard to find a clean car under 25K
Decent Projects sell anywhere from 10-20K
Total resto cars under 10K
Options such as 4 speeds, power windows, air (especially on verts), rallye dashes, 15in wheels help the price higher.
Modified, non-original drivetrains, incorrect parts and bodywork help the price lower!
Private generally seems better (lower) then dealer prices.
I have been tracking these cars over the last 5 yrs. This is roughly what I found based on actual sales.
Just my own,unscientific research!