Several points:
The Twins are not Tampa, Boston or Chicago
We agree that overpayment is to be avoided; always know when to walk away
There are always going to be trades that work well for one or both parties (the Twins appear to have gotten the better of the Graterol - Maeda deal, for example, at least in the short term), but they are inherently riskier than re-signing an asset already in the organization simply because of greater number of unknowns
In terms of WHIP, H9 and BB9 this is Berrios' best season ever, why should the ball club walk away from that?
Shopping himself is a calculated risk for Berrios, as well; he might get the money he wants, but money is not everything
Most important, when trading away an asset, regardless of what is received, have an understudy ready to grow into the role; the hard fact is that the Twins do not have another pitcher that could do as much for the club's performance as Berrios has done since 2017, were there currently someone with the organization already at his 2017 - 2018 level then it might make sense to deal him, but there is not
Bottom line: Make him a generous offer that acknowledges his performance and his upside, and build around him; if he pushes it back across the desk then know it wasn't meant to be - deal him for prospects and let him be another club's problem