All-Pro caliber DE in this market is easily worth that. If we gave him that kind of money, you'd be acting like you don't understand how anyone could question it.
My feeling is that there are only a few defenders worth $30 million per year. Off the top of my head, I can think of ten players at his position who are better than him, so it feels like an overpay.
The Giants pulled off a great trade, getting him for a second and a fifth. Maybe they felt that they had to get it done, given how little they had to give up to get him.
Burns is not good against the run, and PFF ranks him 37th among edge defenders. As a comparison, Boye Mafe is ranked 38th. The Vikings got Van Ginkel (PFF #7) for $10 million and Greenard (PFF #30) for $18 million per year. Or two pretty good edge defenders for less than Burns.