Early in a round, the best purchase you can make is Up'N Atoms. Once you spawn in the map, head to the far left and enter the small office, then turn on the power. Exit the office and make your way to the right to find the perk machine. You can purchase the Up'N Atom to revive yourself in single-player (the cost is $500), or to more quickly revive fallen allies in multiplayer ($1,200).