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...
"Absence of evidence is not evidence of absence. - Carl Sagan