I challenge you to prove that Dion Phaneuf is not a 6.5M defenceman on the open market.
Here's a pretty decent list of the top 30 defensemen in the NHL from a recent HF boards thread. Dion is probably somewhere between #20 and #30 I would say.
#1 - Shea Weber - Nashville Predators
Cap Hit = $7.8M, Years in NHL = 7
#2 - Zdeno Chara - Boston Bruins
Cap Hit = $6.9M, Years in NHL = 14
#3 - Erik Karlsson - Ottawa Senators
Cap Hit = $6.5M, Years in NHL = 3
#4 - Alex Pietrangelo - St. Louis Blues
Cap Hit = $3.2M, Years in NHL = 4
#5 - Ryan Suter - Nashville Predators
Cap Hit = $7.5M, Years in NHL = 7
#6 - Nicklas Lidstrom - Detroit Red Wings (retired)
Cap Hit = $6.2M, Years in NHL = 20!
#7 - Kris Letang - Pittsburgh Penguins
Cap Hit = $3.5M, Years in NHL = 6
#8 - Chris Pronger - Philadelphia Flyers
Cap Hit = $4.9M, Years in NHL = 17
#9 - Drew Doughty - Los Angeles Kings
Cap Hit = $7.0M, Years in NHL = 4
#10 - Duncan Keith - Chicago Blackhawks
Cap Hit = $5.5M, Years in NHL = 7
#11 - Keith Yandle - Phoenix Coyotes
Cap Hit = $5.3M, Years in NHL = 6
#12 - Dan Boyle - San Jose Sharks
Cap Hit = $6.7M, Years in NHL = 14
#13 - Dustin Byfuglien - Winnipeg Jets
Cap Hit = $5.2M, Years in NHL = 7
#14 - Alexander Edler - Vancouver Canucks
Cap Hit = $3.3M, Years in NHL = 6
#15 - Brent Seabrook - Chicago Blackhawks
Cap Hit = $5.8M, Years in NHL = 7
#16 - Brian Campbell - Florida Panthers
Cap Hit = $7.1M, Years in NHL = 13
#17 - PK Subban - Montreal Canadiens
Cap Hit = $2.9M, Years in NHL = 3
#18 - Tobias Enstrom - Winnipeg Jets
Cap Hit = $3.8M, Years in NHL = 5
#19 - Mike Green - New York Rangers
Cap Hit = $7.8M, Years in NHL = 7
#20 - Kimmo Timonen - Philadelphia Flyers
Cap Hit = $6.3M, Years in NHL = 13
#21 - Michael Del Zotto - New York Rangers
Cap Hit = $2.6M, Years in NHL = 3
#22 - Tyler Myers - Buffalo Sabres
Cap Hit = $5.5M, Years in NHL = 3
#23 - Andrei Markov - Montreal Canadiens
Cap Hit = $5.8M, Years in NHL = 11
#24 - Kevin Shattenkirk - St Louis Blues
Cap Hit = $1.3M, Years in NHL = 3
#25 - Dan Girardi - New York Rangers
Cap Hit = $3.3M, Years in NHL = 6
#26 - Nicklas Kronwall - Detroit Red Wings
Cap Hit = $4.8M, Years in NHL = 8
#27 - Brent Burns - San Jose Sharks
Cap Hit = $5.8M, Years in NHL = 8
#28 - Lubomir Lisnovsky - New York Islanders
Cap Hit = $5.6M, Years in NHL = 12
#29 - Sergei Gonchar - Ottawa Senators
Cap Hit = $5.5M, Years in NHL = 18
#30 - Mark Streit - New York Islanders
Cap Hit = $4.1M, Years in NHL = 6
HM: Giordano, Carle, OEL, Bieksa, etc
For Dion, he's played 8 seasons and has a $6.5M cap hit... closest comparables have to be guys like Byfuglien, Burns, Seabrook, Kronwall, Green (hugely overpaid), etc.
In order for him to be fairly paid, I would say something between $5.0-6.0M cap hit.