The going rate for a good backup QB these days is probably around 3 million or so.
It's an interesting situation, IMO.
For Jackson I don't think there's really going to be a market for him, or that anyone will offer him more than he's already getting.
For the Seahawks things have really changed. Unlike years past they don't really have the cap room to burn millions on a backup QB. Likewise, as the offense has become more centered around Wilson, I think the NEED for a backup QB like Jackson has also declined. Basically, in previous years when the offense centered around Lynch if Wilson got injured the spend for having a guy like Jackson behind him made a lot of sense; Jackson could come in and still give the team a good chance of winning because what was asked of the QB wasn't that much compared to other teams. Now that Wilson is more the focal point of the offense, in a weird way Jackson is less valuable, because if WIlson goes does the team is more likely to go down with him.
All in all I think this is the year for the Hawks to draft or try to develop a backup QB rather than sticking with Jackson. That said, if they burn a 3rd or 4th round pick on a backup QB I'll laugh my @ss off. The team has depth problems (and maybe starter problems too depending on how FA shakes out) and that's just a total waste of a draft pick for them right now, especially when also factoring in that 1) Wilson has no injury history and really does a great job of proteceting his body and 2) in a more QB centered offense the difference between a below average and above average backup doesn't actually matter that much. Much smarter IMO would be to focus on very late picks and UFAs for the backup qb spot (bring in about 3 or 4 developmental guys, say), only give Wilson token snaps during the pre-season and let all those guys battle it out for the backup QB spot.