"I'll give you as much as your black little heart desires, it doesn't bother me," said Lucifer. "Consider that my gift to you. We could have a little partnership, you and I. We help each other. I want to stay one step ahead of the Winchesters, because I'm not stupid. You want blood, and you want to keep me from killing you."
He smiled sweetly. "You keep your castle, your stuff. You have as much of Castiel's blood as you want. You feel all those adorable little human emotions coursing through you, you cry to your heart's content. We do each other favors," he murmured, finally releasing Crowley's shoulder.
Lucifer didn't believe that Crowley would truly accept the offer. Crowley would do what he pleased to get what he wanted, and he'd do whatever would save his own smoke. The Winchesters had numbers on their side, and no matter how powerful Lucifer was, Crowley would side with whoever would keep him alive. Demons didn't have principles.