Cousins is pretty meh. Give him Sean McVay as the OC and a bunch of good receivers and he can darn near crack 5,000 passing yards. Give him Jay Gruden running the show or a crappy Vikings OL and he looks a lot more average.
He's basically only as good as the talent around him. That's the case for a lot of QBs in the league so it's not the dig it might sound like, but he's not a guy I'd be crazy to get either.
The Vikings were correct to pay him whatever he wanted because they seriously looked to be a QB away from the Super Bowl, and even as meh as Cousins is, he was still the best quarterback to hit the market in years.