Sabres didn't overpay, it's straight and fair deal.
Forget where Beaulieu was drafted (2011 17th overall). This is a couple years where he can prove himself, or get binned as a bottom pairing / minimum salary guy that bounces around the league. At age 24, Beaulieu should be clearly establishing himself as a D.
I'd err on the pessimistic side of things that Beaulieu isn't on track to play a lot of important minutes for the Sabres, and filling that hole represents more work.