What is the value of a 1977 S cent without a mint mark?

There can't be such a coin because "S" is the mint mark, indicating the coin was made in San Francisco.

San Francisco only made collectors'-version proof coins in 1977 so a 1977-S cent found in change must have come from a proof set that someone opened and spent. Unfortunately opening a proof set and spending it destroys its value to a collector so your coin is now only worth 1¢