Nasri is the epitome of inconsistency; Arshavin is a very good player, I will grant you that.
Diego is one of a few remaining players of a dying breed, a true playmaker, so paying what we paid for him is good value for money; it's not everyday that you can pick up a Zidane for 5 million Euros. Those days are long gone.
Melo's price may be slightly inflated (you can thank Madirid for that), but wait and see what he brings to this team (if he indeed joins); you will be pleasantly surprised.
In today's footballing climate, if you want quality you have to pay for it!