The way people go about this it's like we're a poor club; sure, we shouldn't splash the cash on every single talent we see, but if we buy someone for 40 million and they're not the next Messi that doesn't mean they're not good. In these three seasons, he given us 27 goals and 17 assists in 65 games(
source), which is a decent return considering he's only 24. Sure, he's not the most consistent, but even if he gets 15-20 goals + assists every season for the next four or five years, he'll be good value.