Definitely an overpay for Butler. I'm a proponent of trying to get Butler, but trading all three of Avery, Jae AND the #3 pick is way too much.
Also, some chemistry considerations: Bradley and Durant are apparently good friends (U of Texas connection), while Crowder and Butler are apparently besties (Marquette connection).
I would try to build a package that didn't include either. Try to sell youth for rebuilding with a package that has Smart or Rozier (someone they were interested in the draft) plus picks. Use Amir's contract for salary matching.
Even if Durant _didn't_ sign, you'd still have a hell of a young core (Butler, Isaiah, Jae, Avery) under contract. If you add Durant and a top quality big like Horford, you have a contender to get out of the East for the next few years.