How much thier worth depends on the condition of them.
If the are totally bone stock with no repairs they may fetch upwards of 3k to the rite person that is. Someone who is restoring a Vette, Camaro, or Chevelle would be the one who would pay the most.
If they have been hack ported and have had the typical stud boss repairs they will be worth considerably less.
If you really want to know what they are worth, list them on EBAY.