Boone is a fantastic OL that has excelled at RG for a while and also did a fantastic job at LT when replacing Staley. The man would command a very high trade value on the open market as many teams would be more than happy to agree to another contract after he is traded - he could be a legitimately good LT in the future. I also don't think that he is looking for another million/year - I think he is looking for at least 3 million/year more than his current contract, possibly more.
We also have to consider that Iupati, Kap, Crabtree, and other good players are also looking for a piece of a very limited amount of $$$ we have to go around next year. We will not be able to retain all 4 of these players at their market values. Thus looking at each of their contributions to the team has to be accounted for along with the possible return in not retaining them going forward.
Now for Iupati, Crabtree, and Kap, there is zero value in trading them as they will be free agents next year anyways. They are also not looking at a possibility of holding out for some/all of the season.
This, in my opinion, would mean that the best return for the team would be to try and retain Kap, Crabs, and Iupati while trading Boone to a team willing to restructure or rewrite a new contract to his liking after the trade. There are many teams out there who would gladly pay for him as a guard who can swing as a quality backup tackle. There are many teams out there who would also gladly give him a shot at LT/RT after he manhandled Quinn when stepping in for Staley - especially because if he doesn't pan out as a tackle, he's still in the top 1/4 of guards in the league with the potential to continue to get better and better.
What is the trade value for a very good guard around his prime who may also be a very good tackle? I wouldn't dismiss the possiblity of a conditional 2nd/3rd round pick. I think there would be a good amount of suitors as well. But I would be interested to hear what others think.