If you walk the paths that other collectors don't walk, you can expect to see very reasonable prices. There are not many collectors at small ham fests and local swap meets. Small town antique shops don't get many key collectors passing through. One of the best means for adding keys to your collection is referrals from friends. Ask around at the local ham club.
Word of mouth is one of the best avenues for flushing out keys.
Very, very good prices for keys can be found at estate sales. The odds are long, but the reward is high when you find something. The same goes for antique auctions. Check the manifest. Another good means is to stay in touch with experienced collectors who are well connected.