Not only is a coin cheaper to produce, it can last in circulation much longer than a paper bill can.
Actually, after looking it up, the cost of production for a dollar coin is 9 cents, however, it has much more longevity (standard 1 dollar bill doesn't last very long), so that's where the savings come in.
Nonetheless, I find coins impractical. I barely tolerate cash as it is. If the government is going to force me to carry around a dollar coin, you can bet your house I'm going to only carry $5 notes and above.