Let's say Shelby Miller makes like $6/9/12 million in his arbitration years. That means his actual cost for the next 4 years is $27 million or so. If you add Heyward's excess value (let's ignore Walden and the kinda-prospect), that gives a Braves valuation for Shelby Miller of $54-59 million over 4 years, as a breakeven point for this trade.

It's not outrageous, and he could definitely be worth that. But it's a pretty big bet.

There's also a non-zero chance that Heyward ends up signing an extension with the Cardinals and becoming an MVP candidate, in which case you can probably throw out "excess value" calculations and this will look awful no matter what the numbers are.