I guess if it allows you to win your league this year then pull the trigger, but I definitely don't like it for next year and beyond.
The question I always ask myself is: if I don't keep any of these players can I buy them back at or near their current number?
Without knowing the parameters/nuances of your league my gut tells me:
Braun at $45, probably...may cost me a little more, but it'll be in the ballpark either way.
Greinke at $26, most likely...again, may be inflated based on where he signs, but in the ballpark.
McCutchen at $14, no way in hell. He's in Braun territory now.
So if this were in one of my leagues I'd be dealing one of the best values for two people I wouldn't be keeping...trump card would be whether I think these two are exactly what I need to put me over the top for this year.
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