A sharp smile. They're both so well behaved anyway, of course.

"Pending you don't tell anyone who you're working with, and I never have to deal with the business end, I'll go half with you."

He's not a moron - Severus knows full well the value of someone skilled in these arts, as well of the value of silence. He knows Lucius Malfoy isn't going to accept anything less than that, and so he's not even going to try to work him down to something.