Details: The format is commander. However, your deck is limited to a $15 budget. The prices are based off of "TCG player mid" prices. The condition of your card is not taken into account. (i.e. If you bought a "Heavily Played/Damaged Garruk, Primal Hunter" for $3, it doesn't count as $3 to your deck. The price of the Garruk is based of it's "Near Mint TCG Mid" price.) Overall, the total value of your deck is to remain below $15, not including basic lands.
As the league the progresses, winners get a larger budget to work with. (This larger budget is not to exceed $20 and is gradually earned through progression in the league.)