I'm not sure about other sports, but the football ones are listed in the annual Football Beckett standard price guide (the big book), in the "multi-sport" section at the back. In NRMT-MT condition, the guide lists them anywhere from $1.50 to $20 each, depending on who they are and from which set.
The later sets are more scarce since most subscribers dropped out before then.
Football collector 1948-1995, Rams oddball cards & memorabilia, Diamond match. Cataloging all those pesky, unlisted 1963 Topps football color variationsUpdated 2/13/05
I collect Vintage Cards, Commemorative Sets, and way too many vintage and modern player collections in Baseball (180 players), Football (175 players), and Basketball (87 players). Also have a Dallas Cowboy team collection.
Comments
The later sets are more scarce since most subscribers dropped out before then.
Cataloging all those pesky, unlisted 1963 Topps football color variations Updated 2/13/05
I need some, so I may be in contact via PM.