Ed, I don't think that anyone will give a first round pick for Leonard Davis. A first-round pick, even in the 20s, it too valuable today from both a financial and future-benefit perspective.
That said, I think that it is absolutely imperative that the Cardinals retain the services of Leonard Davis until at least the 2008 season. The Cards have no one that can replace Davis even for a handful of games right now. There's no one who's going to be able to replace even 50% of Davis's production in free agency. You have to franchise him because the front office did a horrible job of putting the pieces in place to possibly replace him (as Rod Graves failed to do at WR following the departures of Boston, Sanders, and Jenkins and at cornerback after Starks and Hill left--again, killer re-signing, Bidwills).
Leonard Davis did a great job against Merriman and his partner on the other side last week. He's been clearing holes for Edge to run through. I know that $11 million is a big number to swallow (pun not intended, but noted), but I think that Davis is just about as good as Bryant McKinnie in Minny, and look at the deal that he got.
If this team is going to get better, you can't keep using the draft to plug holes that you made yourself. I agree that Leonard Davis isn't a "core" player on this team. But letting him walk with nothing to show for it and no one to step in would be a step backward for the entire franchise.