Oslo is the most expensive European city to buy Facebook advertising, according to analysis from marketing software firm Kenshoo. For Newsfeed placements on desktop and mobile, Olso ranked the most expensive city at $0.73 a click-through. In second place was Vatican City at $0.70, followed by Helsinki ($0.57), Berlin (0.54) and Bern (0.53). London was ranked ninth - costing $0.48 per conversion.