Interesting article on FG calculating Stanton's trade value:
http://www.fangraphs.com/blogs/assessing-the-trade-value-of-giancarlo-stanton/
The surplus value calculation is something we've all seen and done ourselves a million times, so that part isn't particularly interesting.
The interesting part is that $17M/WAR value the author assigns to Stanton's projected remaining 2017 WAR. This extra money is what we refer to as the "contender's premium". In Stanton's case, the author speculates a contending team should be willing to pay an extra $18M for Stanton just because they are relatively certain they will make the post season this year, and will use him in post season games NOW.
That is a significant cost, as $18M is about the value of a Top 100 position prospect or a Top 60 pitching prospect. The author doesn't offer any justification for arriving at that 2x value, but it logically passes the sniff test.
This is why the Braves would be foolish to add a long term piece at the trade deadline when they are not competing for a playoff spot. Paying the contender premium when you aren't a contender is a foolish use of resources.