There are 200,000,000,000 (or so) pennies in circulation.
To buy them all up, for a nickle a apiece, would cost you 4 cents (the nickle less the penny you get in return) times 2oo billion, or 8 billion dollars.
And then, to buy back all of the nickels for 25 cents...well, why waste time on these boards if you have that kind of scratch...