I don't believe the Bonneville had an option for anything other than a 421ci engine in 1966. And there were 3 of those available in 338hp, 356hp, and 376hp ratings.
Selecting a 2-door hardtop with the HO engine (376hp 421ci), and air conditioning, the HIGHEST you should pay is $9900 in excellent condition. Low retail is $3450.
That should get you in the ballpark. Assume the other two horsepower ratings drop you about 15% in price each step down.
(according to NADA)