We'd never give up that much for Butler- as in we'd never give up that much that's important to our current core. In my opinion we'd rather lose one core defender (probably Bradley because he'd be replaced by Butler) and keep Crowder/Smart/IT. Smart could also be traded but I think they'd rather keep him long term vs Bradley.
We'd give up the Nets pick and maybe even Smart if we had to, but we would need to keep at least Crowder+Bradley+IT to make a championship run. I've said it before a few times, but I think if we are going after Butler it's something like:
16' Nets pick
18' Nets pick
16' Mavs pick
Bradley
Jerebko (filler)
and one of Rozier (who the Bulls wanted to draft and would probably ask for), Hunter or Mickey.