US Mint to issue $1 Grant coin. Meh. His spouse's coin? Wow!

For investors, it usually pays to wait for the better half.

On May 19, the US Mint has announced it will release the latest of its $1 presidential coins. The 18^th coin is reserved for the 18^th president and commander of the Union Army during the Civil War, Ulysses S. Grant. It's a nice commemoration, but a lousy investment.

Demand for presidential coins has dropped. When the first presidential coin was released – George Washington in February 2007 – about 340 million coins were minted. (Each coin is minted until the next one is released, and the number minted depends on demand.) By the time Andrew Johnson rolled around earlier this year, only about 73 million were minted. Even popular Lincoln got only 97 million.

Then there's the shrinking value of the dollar to consider. If you'd bought that $1 Washington coin back in 2007, today it would be worth only about 89 cents against a broad basket of foreign currencies.

IN PICTURES: Gold’s journey

If you want to invest in White House occupants, pick the spouse. When the US Mint issued its initial “First Spouse” $10 gold coins in 2007, the Martha Washington proof coin was selling for about $430. It's half an ounce of gold, after all.

Since then, gold prices have skyrocketed and the spouse coins have nearly doubled in value. A Martha Washington would fetch about $848 today if you were to buy it from a dealer, according to Blanchard and Co., a gold-coin dealer based in New Orleans.

So when the newest spouse coin is released next month, which will feature Eliza Johnson (wife of Andrew), it may be worth filling a sack with $1 Washingtons and trading them in for something that will really hold her value.