There are different ways to look at what Cassel's "value" is.
The Pats put the franchise tag on him and will pay him $14.6M. That is a major investment, and also indicates how highly they value him, and it is not something that you give away carelessly. I think that a first round pick is the minimum starting point regarding compensation for a resource valued that highly.
Even if Cassel does not turn out to be a true "franchise" QB, the value of a solid, better-than-average, consistent long-term starting QB for a team should be at least worth a mid first round pick plus a conditional pick.
For a team that is on the verge of contending (such as Minnesota and possibly Chicago or Tampa Bay), Cassel could turn them into a consistent long-term contender and possibly position them to go far in the postseason. That has got to be worth almost any price. Tampa gave 2 firsts (and I think 2 seconds as well) for Jon Gruden in 2001 and got a SB out of it. Pittsburgh went from a solid team to a perennial contender as soon as they got Rothlisberger with the #11 pick.
For a team that has been at or near the bottom of the barrel for some time, such as Detroit, KC, or SF, having a solid long-term QB who could help restore them to respectability would likely be more at least a 1st round pick. KC or SF could pick Stafford or Sanchez without anyone questioning it, yet statistically these players have far less chance of succeeding than Cassel. For every Rothlisberger, Carson Palmer and Jay Cutler there is at least one Michael Vick, David Carr, Joey Harrington, Byron Leftwich, etc.
Since no one truly knows how good Cassel would be, I would set an initial reasonable price somewhere in the middle, something like a mid-1st round pick plus either a 2nd round pick this year or a conditional pick which could potentially escalate to a 1st round pick next year depending on performance and team outcome.