What is the 1973 bicentennial commemorative medal worth?

It is not gold. Circulation-issue Bicentennial quarters were only struck in cupronickel, like all other circulation quarters since 1965. In fact, no gold coins have been struck for circulation since 1933.

A lot of private companies put a thin plating of gold on ordinary quarters and sell them at significant markups. You can find the same thing done to the current series of state quarters. However the plating is generally so thin that it only adds a couple of cents to the coin's actual value.

In addition, from a numismatic standpoint these coins are considered to be novelty items because they were altered outside of the mint, so they do not have any special value to mainstream coin collectors.