If you include Michalek's salary minus the replacement player, we are saving about 4 mil in real dollars next year by having Phaneuf help the team rather than Cowen hurt the team while Michalek nurses a broken whatever he ends up hurting and Greening plays in Bingo.
Its the season after that which costs us 7 mil for Phaneuf's service after which his salary continues to decline.
In fact, buying out Greening and Cowen would have had us paying them a salary for twice as long (but only a portion of it). So, buying them out would have cost us 3.6 mil for the right to not have them grace our ice for two seasons.
So, can you get a UFA D that provides as much as Phaneuf does, for essentially nothing next season (~3.2 for converting Michalek to Dzingel or other prospect, and the buyout cost of Greening and Cowen at 3.6 totaling), and then ~3.4 the season after? Probably not. In 2018-19, when the buyouts would be clear, we'd be comparing 6.5 in salary to available UFA, at which point you can probably find better value, same goes for 19-20. At 5.5 in 20-21 I think it'll be dependant on how Phaneuf's body is holding up.