I think its really hard to determine a coaches value. With players, they either make plays or they dont make plays. They have the stats or they dont. Its easy to see what level of production you are getting. With coaches, well, I guess wins would be the only concrete standard. After that, alot of it boils down to how well they can sale themselves. Whether it be other player and coaches testimonials, or the general fan feel for a guy. I just dont know. How much credit can you give that coach for building a contender vs the rest of the staff and GM?
The other difficult question is, who would replace him? Would you know ahead of time prior to making the trade? Unlike players, theres only one head coach. However hard it is to quantify his impact on the team, its even more difficult to know if the next guy up could even compare in the long run.
Back on the topic at hand. As of right now, I wouldnt even begin negotiations because the team is built and ready. We all know this, so there is no reason to change it. But, if I had to guess a correct value for a coach (being the key question), I would say a all-pro player (that fits a need) and 2 first rounds would sound about right.