No cheats are available for Dirge of Cerberus -Final Fantasy VII-. However, there are many unlockable easter eggs that one can find outside of the main action.
Before you fight Nero the Sable in the Chapter 10, keep an eye out for some barrels above you as you run on through. There will be four barrels that blow up in a line. One of those barrels contains one of Final Fantasy's best known hidden character "Cactuar". As it falls down to the ground it will start running away and as soon as it gets to the corner it hops on up and disappears. A second appearance occurs on the WRO Causeway. At the top of the right hand tower from where you start the scene, look to the very top of the antenna. You'll see Cactuar at the top, doing that weird little Cactuar dance.
In Extra hard mode, after protecting the little boy that leads you to the warehouse, you will kill a guard in the final house the kid goes to. That guard drops a keycard. Pick up the keycard and it will do a cut-scene and put you outside the house.
You have to turn around and go through the door you just exited. You will see a lone item sitting under the staircase. Pick it up. It is called the "Toy Gun". Upgrade for 15,OOO gil to get the "Silver Toy Gun", and again for 35,OOO to get the "Golden Toy Gun". A final upgrade for 2OO,OOO gil will unlock the "Ultima Weapon".
Portrait of Cloud
In a mission where you are in Edge for the first time (while you are rescuing the little boy) you will jump out the window in a cut scene at one point. This is at the part where the boy is showing you where the cardkey is. Look just under the building you jumped out of, and you will see a little black picture of Cloud with his sword over his back. This is easier to see with a gun with a sniper scope.
In the first level, after meeting Reeve and leaving the house (the start of mission 'Assist the WRO') the 5 to 6 Deepground soldiers you fight there will continually respawn until you head toward the gates with the Mako points. Between selling the potions they drop, and the experience you can convert to gold (or let them kill you, add to total, then lather-rinse-repeat) you can build as much Gil as you want. Makes buying the final weapon upgrades much easier, as well as maxing out one or all three of every type of gun.
In chapter 10 - Shinra's Dark Secret, in the map before you go to fight Nero there is an optional Stage Mission where you must defeat 100 enemies. Stock up on all sorts of healing items and ammo, then do this mission. Some of the Deepground Soldiers will drop Gil, but the 97th, 98th, and 99th soldiers each drop 8,000 Gil and the 100th solider drops a Gold Moogle Doll which can be sold for 35,000 Gil. You can do this mission over and over stocking up enormous amounts of EXP and Gil.