My take:
Teheran projects as a 3.5 WAR per season player over the rest of his contract (through 2020 with the option). Or a total of about 16 wins. During that time he will be paid $39M. At the going rate of about 7M/win that works out to about 5.5 wins. So he is a player worth 16 wins being paid a salary worth about 5.5 wins. So his expected surplus value is 10 wins.
There have been various analyses of what a top prospect is worth. Here is one study:
http://www.thepointofpittsburgh.com/mlb-prospect-surplus-values-2016-updated-edition/
It finds that a Top 10 prospect has a surplus value of about 10 wins.
So it appears to me that fair value for Teheran would be a one-for-one trade for someone like Moncada.
Of course, I want to win any trade involving a very valuable asset for Teheran, though I am realistic enough to know that a win of the order of magnitude of the Shelby Miller trade is unlikely. I would be looking for Moncada plus an asset with a surplus value of about 3 WAR. That would be my price in any talks with the Red Sox. There isn't any need to play games with them. Have a bottom line and walk away if they won't meet it.