For the US period you are interested in, the stamps are not expensive but some dealers will charge a lot for them.
You may also consider stamp forums that have sections where collectors sell approvals. You can pick many stamps up near face value and you get to see a picture of them before you buy.
For example, Mystic sells a mint version of 1950 3c American Bankers Association (Scott 987) in Mint for 40 cents but this stamp is common and like to get near face.
If you use catalog value, this package I received has stamps has good catalogue. The market is flooded with 40's and later stampss so why pay too much.
