The return on a trade would be draft picks, and even first-round picks are a roll of the dice. I think the important question is the one bolded above: he might take a bit less to extend with the Patriots knowing it's a quality program he's familiar with in which he will be groomed as Brady's successor. Like I said, the Patriots might have to overspend on the QB position for a year or two but it just might be worth the sacrifice if Garoppolo is as good as I think he might be.
People are obsessed with outcomes, but process also matters. The return on a trade would be good even if the picks didn't pan out.
As an example, let's say you bought an old jersey online for $25. Then it gains a tremendous amount of value (say you find a signature on it or something) and then sell it for $200. Then you blow the $200 on lottery tickets and win nothing.
It was still a good deal to sell it for the price you did, regardless of the outcome of what you spent the money on. They are two separate transactions.
It depends on how stupid/desperate a team like, say Cleveland is. It could be anywhere from trading first round picks from 32 to 1 over all, to a 1 and a two. (Before anyone craps on the 1 and a two being stupid, we are talking Cleveland )
Desperation can obviously drive up that value, and yes Bradford got a 1st, but nobody thinks he's worth that, even Minnesota. They're doing a different calculation of their contending window and feel it's not worth sacrificing one of the last Peterson years with a solid contender. But that calculation has way more to do with everyone else on the roster than Bradford's actual worth.
Personally, I think Jimmy is worth a lot and yet I don't think teams will give up a lot. Maybe a 1st, but multiple 1sts seems way too much. One of the benefits of drafting a young QB is having 4 or 5 years of really good QB play for a fraction of the cost on your salary cap. Trading for Jimmy will only give 1 year of that value.
And while Cleveland is historically stupid and cursed, their new guys are willing to suck for a while. They're not desperate, and it is not business as usual in Cleveland. They are maximizing their draft pick value and I could see the analytics side driving them to draft a QB with that 1st rather than trade it for a QB. You can never say never with Cleveland, but I would be surprised if they were going to trade a lot.
Where it might get interesting is in the coaching carousel, especially if Josh goes out on his own. I remember hearing he made an offer for Cassel (and a better one than the Chiefs did, but BB already agreed with Pioli). I'd hate to lose Josh, but if he's gone anyways, maybe he can flip us a nice bounty to take Jimmy with him.