I ran this through the simulator at baseballtradevalues.com, but without the money since it can only handle increments of $1 million.
It basically says the Mets are getting screwed here and the Yankees are making out like bandits (and remember, that is before the money was added). It was an equal trade for the Brewers, at least in terms of surplus value. Now, they might have different feeling about using their best trade asset to get some "insurance" for two holes they are filling and prefer to just fill another hole.
Per BTV, the Yankees are acquiring $89mill in surplus value while only giving up $45mill. The Mets are acquiring $62mill in surplus value while giving up $104mill. You might be underestimating what it will take to get Syndergaard, who on his own is worth an estimated $75.7mill in surplus value.
And then there is the whole question of why the Mets would acquire an asset like Hader, who represents an immediate fix, and the go and deal an asset like Syndergaard, which makes it much more difficult to take advantage of immediate fixes. This is the kind of thing simple trade values cannot catch, but the logic is counter-intuitive for the Mets. Especially since this team is trying to win and already lost Zack Wheeler in the rotation. Losing Syndergaard just makes their whole season taht much more difficult
This trade does become a lot more equal in terms of trade value if the Yankees are willing to take back the contracts of Cespedes and Familia, but that doesn't solve the logic problem for the Mets.
It also works out even better if the Yankees just take Syndergaard and Cespedes and let the Mets keep Diaz, and have their power combo to close out games, assuming they have any starters left capable of giving them a lead. That trade was accepted by the simulator. Or if the Yankees don't want Cespedes (they don't), they could also give up Deivi Garcia and either Tauchman or Antonio Cabello to the Mets, although neither really fills their immediate need for a CF...