Did you just beat the game? How much of the Crystarium have you mastered for everyone? The best combination of both gil-making and CP farming is fighting Adamantortoises, but it might be a bit slow at first since at first you'll probably need to use a Summon to knock it down. You should probably start with the Adamantortoises because they're a bit weaker than the Adamantoises (they can be differentiated by somewhat shorter tusks). You should do Mission 55 for the Growth Egg to double CP as soon as possible; it will require you using Vanille for Death and likely at least a few attempts, but it's worth it (see my half-finished Missions guide for more details).

An alternate place for just CP farming is the place where the King Behemoth is fighting the other big monster in that narrow path by one of the Steppe's waystones. It should take only 10-15 seconds and gives you 13200 CP with the Growth Egg. For just gil, the Sacrifices (white Cie'th things) in the chapter 13 dungeon drop Perfumes which can be sold for a fair amount.

DO NOT buy a Trap, unless you get to at least 3.5 million gil without getting any at all from the Adamantoises/tortoises. That's the amount it would cost you to buy a Trap and fully upgrade one of Vanille's or Fang's weapons to dismantle it for 3 Traps. You will probably get one by then.