You need 200 Mercenary Commendations as well as 200 Warzone Commendations for one Champion PvP bag, purchasable from the PvP Items vendor next to the quest terminal in the Combat Training section of the fleet. Mercenary commendations are purchased from this vendor with Warzone comms at a rate of 3:1, so for each Champion bag you'll need a total of 800 Warzone commendations. Buying the bags is more of a supplement to the bags you get as rewards from the Weekly/Daily pvp quests. So, basically, do the PvP dailies every day for one Champ bag, the weeklies for 3 Champ Bags, and supplement these with the bags you purchase with the commendations you get from doing all the warzones. tl;dr: PvP your heart out.