Well, JPP and Gronk are both 29-year-old "superstars" with significant injury histories, coming off very productive seasons, and carrying ~$11M salaries that will be paid by their "new" teams - who might get as little as another two seasons out of them.
The Giants got a 3rd and moved up in the 4th in exchange for JPP. Because Gronk is an offensive weapon and, arguably, "better" at his position than JPP, I'm guessing that the Pats might see an offer of a 2nd and a move up in the 3rd.
I'm not saying that the Pats should accept that, I'm merely suggesting that it's a realistic estimate of what the market will bear - especially since "the market" is likely to be limited to the 16 NFC teams (it's possible, but doubtful, that the Pats would trade him in the conference). But I seriously doubt that an offer of a 1st rounder for Gronk is anything like "automatic".
As for Cooks ...
He's 24, consistently very productive, and with the bulk of his career ahead of him. And his only "injury history" is a broken thumb he suffered as a rookie. He's also carrying a cap hit of only $8.5M at the moment.
I don't know of a recent trade that might be considered comparable. The Jarvis Landry trade really doesn't apply very well, since Miami was under the gun to shed that $16M cap expense - which will now be absorbed by the Browns.
While I expect that offers for Cooks would be considerably more than the 4th and 7th that the Browns traded for Landry, I also strongly doubt that Cooks would fetch a 1st-rounder. Maybe a 2nd.