Essien cost ten times more because his form for Lyon had been so impressive, whereas Mikel wasn't really pulling up any trees for Sociedad. Then you add the fact that moneybags Chelski and Arsenal and Man Utd were all bidding for Essien and the price inflates even further. Hell, most likely half of Europe was bidding for him, whereas not many - if any - of Europe's big guns would have been bidding for Arteta.
Football is a supply and demand market, and it seems that players the ilk of Essien are in short supply. Arteta has been great for us, but even then not so much this year, whereas when most people look at Essien they realise this fella can be world class for most teams.
Edit: I'm not saying that I think he is one of the top 50 transfers of all time btw, just that I can see why he would be called that. In fact, I'm more about saying that it doesn't make much sense to compare his and Arteta's transfers to each other. Perhaps compare Essien to Chelsea with Veron to Man Utd? That would seem a tad fairer based on the particular situations involved.