How many teams are in the league? That will help set the value of each round. Also, do you know what slot you'll be drafting from at all? Forte for a first round pick sounds good if you pick last out of 12+ teams, not so hot if you pick 3rd. Barring that info, I think you have to keep Cam at that value. I really like Mathews for a 3rd - my only concern is if Norv Turner is out, what type of offense will the new coach want to run? A new HC would also impact a VJax, although who knows if he'll be playing in SD next year or not. Rumor mill also says the GM in SD could be on the way out, and that would change how the team negotiates with players - so that might be a good sign for VJax staying.
I think I lean away from keeping Forte - getting him for a first round pick isn't a steal of a value, whereas I do think there is good value in Cam, Mathews, VJax, and Graham. Again, Cam is a no brainer keeper at that price tag. And I really want to find a way to keep Graham. In a league that counts TEs as just another WR, I think Graham would be a 2nd or 3rd round draft pick. And as much as I like Mathews, his value doesn't seem quite as strong to me as VJax (if he's with the Chargers). So, I'd go Cam, VJax and Graham. That gives you a QB, TE, and WR all at around a 3-4th round value, while paying a 7th, 12th, and 15th round pick. You could then spend your first 3 rounds picking up RBs and a WR, 4th round on a flex player, and could use your 5th round pick to grab your first true backup.
10 team ppr keeper/cap league
QB: Stafford, Cutler
RB: Foster, Rice, Sproles, Jacquizz, Tate, Wilson
WR: Green, Austin, Decker, Colston, Ogletree
Def: Green Bay