Come on, Cillessen is not that bad. He's usually really consistent, but not spectacular. His reflexes aren't special, but he's usually really composed and solid, and more than decent with the ball at his feet. Lately he's been out of sorts a bit, but he should be a decent back-up imo. He fits the job description as a sweeper keeper far better than Diego Alves imo. Alves is far superior when it comes to reflexes and penalty saves and such, though.
I also disagree with him being bad in the 2014 World Cup, he was really solid there, apart from the match against Mexico maybe (probably due to the scorching heat) and in the penalty series against Argentina (think Oblak: he's really bad at saving penalties, horrible even). Against Spain and Costa Rica, for instance, though, he made some crucial saves, and he was really good in the normal part of the game against Argentina.
All in all, 10 million for Cillessen would be a decent transfer imo. Expect a solid, if not unspectacular, keeper. His attitude won't frighten any striker, but he was crucial in Ajax' title runs (4 in a row) and usually quite solid in the national team as well. That said, I'd say his progress has stalled a bit since the World Cup, but if he'd accept a role as a back-up, we could do a lot worse, especially for the small amount of money we'd have to pay for him. Making a 10+ million profit on Bravo whilst signing a younger, albeit weaker replacement, would be a decent bit of business imo. Let's be frank: Bravo is too good to be a back-up, Cillessen fits the profile, and we make a profit. Don't panic just yet.