Somewhere in between 18-24 million over 3 years, structured to escape rather easily after one year would work.
As much faith as I have in our ability to find talent in the draft (and we certainly still could draft a DE), picking up Jared Allen would allow Pete and John more flexibility in the draft to find "their" guys.
I highly anticipate that we will go hard after offensive linemen, a corner, a wider receiver, and possibly a linebacker to eventually replace KJ and/or Malcolm.
The only downfall is that (though most expect Earl to be extended), this would likely decrease the likelihood that they are able to negotiate with Sherman (maybe I'm wrong here).