This is so painful, we get this bum for $12 a year and the Suns get Tyson Chandler for $14m?
They are not even on the same planet talent wise. 
I think a lot of this was based on the overpay on what is essentially a 1 year deal. I'm sure the C's probably could have gotten Johnson in the 6-8 million per year range if they were willing to give him the 4 years that Phoenix is giving Chandler.
But much like a few years ago when the Sox signed guys like Victorino, Gomes, and Napoli to lucrative short term deals, the Celtics are overpaying in the short term to maintain financial flexibility in the long term.