Considering what Barca paid, it's to be expected. If he has a great season and proves to be a quality player, €15m is a more affordable price than if he stayed at Barca, played a great season, and than we try to sign him.
The only problem I have is his loan fee. Hopefully it's reasonable. If it's €3m like Mark mentioned earlier, it's too much. In fact, if the option is really worth €15m, the loan fee should be no more than €1m, so Barca recoup what they paid for him. Barca slapping an €18m (loan fee + option to buy) price tag on him is ridicolous, since he did nothing during his time at Barca for his value to increase.