I think Jimmy's worth will be determined almost entirely by his on-field performance, with little to no regard for where he was originally drafted or the 'studying under Brady/Belichick' factor. Cassel/Hoyer/Mallett have proved that isn't exactly a magic bullet.
The "he was worth X before three years of grooming, so he must be worth more than X now" reasoning fails to take cost control into account. Part of a player's value when drafted if that you have him locked up for four years to evaluate him and get cheap production out of him. Conversely, anyone going into the last year of his contract is going to be worth less because that's no longer the case. Any team trading for him is paying for him twice: in the draft capital they give up, and then again in the long-term extension they give him. That's why Matt Cassel wasn't able to fetch a first after 2008.
That said, I could definitely see Garoppolo being worth a 1st if next year's QB trade/FA market is in line with this past offseason's. 3 more starts like the last one and it becomes pretty easy to argue he's worth as much as Osweiler and Bradford.
My dream scenario is that we get exactly what the Eagles got for Bradford: a 1st and a 4th. Also, coincidentally, exactly what Goodell stole from us for Deflategate. At that point, the narrative becomes clear: by suspending Brady, Goodell forced the Patriots to start Garoppolo for 4 games, giving him the audition reel which enabled the Patriots to recoup the draft picks that he took away. Add in going 3-1 or 4-0 in Brady's absence, and the net on-field effect of Deflategate becomes literally nothing.