To a certain degree that's a seperate issue, as value for money and comparison with expectations are part of how you determine whether someone is a flop. In the cases of Ozil, Bale and di Maria, the issue has been consistency. Bale was getting much less flak this time last year, for example, and I think it's too early to call him a flop yet. Ozil has shown in spurts, but people criticize him because he's had spells of being poor (regardless of price). Di Maria has pretty much just been poor after the first few weeks, but people weren't moaning too much about the price at the start.
Coutinho gets praised because he was probably our best player last season, despite the stats and regardless of price. That he was cheap is a bonus. Sahin only cost the loan fee, but he was criticized becuase he didn't live up to expectations. Balotelli, Borini and Lambert get a lot of stick, even though none of them were particularly expensive either, and you could perhaps say that ll have lived up to the expectations...