What is the value of a 1974 aluminum penny?

The coins are technically illegal to own, but if found their value would likely be in the hundreds of thousands of dollars.

The Mint proposed changing the cent's composition to aluminum because the price of copper had risen to the point where a cent contained more than a penny's worth of that metal. Well over a million aluminum cents were struck in anticipation of a full production run, but the price of copper fell and all aluminum cents were to have been destroyed. However many of them had also been distributed to members of Congress for their approval and not all were returned.

There's much more information at the Related Link.