Dealing with Contreras: The player is sent to Miguel's Pawn Shop to find Price, Contreras' contact, but instead Keller is there. Talk to him and report back to Contreras. He'll ask you to kill Keller, but you can convince Contreras to work with him, Barter check of 70 or Speech check of 80. Whichever option you took go back to Keller and either kill him or convince him to work with Contreras.
pc After talking with Keller and passing the INT/PER check, if you choose to attack and kill Keller after he says you're under arrest, when you return to Contreras the result and dialog option will be the same as if you kept Keller alive and bribed him to play along and keep his mouth shut about the chem trafficking. If you kill Keller in this order you will not take a hit to your reputation with the NCR, and you will gain access to the stash and This Machine. Although it disables the ability to turn him in to Lt. Boyd (This occurs as of patch #2).
xbox360 Sometimes, even if you kill him without NCR reputation loss, the NCR will be hostile towards you. A fix for this is to reload to the save after you killed him but before the NCR went hostile and travel to the Strip.
ps3 If you say that you will rat out Contreras, and then kill Keller, you will lose no reputation with the NCR.
xbox360 Upon passing the Speech check to convince Keller you are merely seeking Price to purchase from him, you will gain no XP for doing so, unlike every other Speech check in the game.