So look at the going prices:
http://www.cargurus.com/Cars/l-Used-2013-BMW-M5-c23342
Even without your Montana tax savings, I am still ahead of the game. Now add the convenience of the trade process. Win all around.
Not going to fool myself into thinking anyone will pay me MORE than $93-$94 for my car. If they paid the SAME price, I would sooner trade to avoid potential fraud and hassles.
As I said-every deal stands on its own. I know this game backwards and forwards and I play it well.