Extras include soda (either the sticker plan or individually), alcoholic drinks, spa treatments, arcade cards, excursions, souveniers, and tips....
Our family of 4 just did a 4 day cruise. Dh and I had a drink or two a day, had the soda plan, bought several souveniers, rented snorkel equipment and floats on CC and tipped well---our stateroom bill was, I believe, just over $600. ($200 of that was tips)
We did not do any formal excursions or spa treatments.
So it was reasonable, IMHO. Compared to our bill after 12 days at the World, it was a STEAL!
Hope this gives you an idea of extra costs.