Minecraft coins can only be purchased with real money. See below for the cost of Minecraft coin packs in USD:
$1.99 = 320 Minecoins
$5.99 = 1020 Minecoins
$9.99 = 1720 Minecoins
$19.99 = 3500 Minecoins
$49.99 = 8800 Minecoins