I'm looking at the red book, and it shows a value of $6 for regular clad bicentennial quarters from the D and P mints in MS-65 condition, while it shows $4 for the clad proof minted in the S mint in PF-65 condition. It also shows the silver business strike quarter at $7 in MS-65, only a dollar more than the clad coins. Why is this? Were there very few bicentennial quarters that were minted in MS-65 condition? I know the red book isn't totally realistic on prices.
The reason you will see higher prices anywhere including the Red Book for MS-65 business strikes than PR-65 is that the process is different for striking each type. A lot more PR65 examples are available than MS-65 business strikes. There just were not as many MS-65 quality business strikes to start with.