When I lived in New York, I knew first hand how much it cost to live there, but I never realized how much it costs to visit.
Of course, there are a lot of ways to control your costs when you visit a place. Stay further away from the city center, stay in a cheap hotel or hostel, take city transit instead of cabs to get around and to see the city instead of tour buses, and eat where the locals eat (I bought lunch every day from the street carts in Manhattan).
These prices are in British Pounds, so knock about 25% off the cost to estimate in USD.
I was surprised to see that Oslo was only #10 because we found Norway to be extremely expensive. The costs to visit the rest of the countries in the world are listed on Hoppa.