What is the definition of 外省?
外省(outside provinces / other provinces) refers to all provinces other than 本省(own province) - the province you are currently residing. In this case, 外省 means any province other than Île-de-France where Paris is located in.

Paris is a city, so "巴黎10W+" refers to the living cost in Paris only. The quote also mentioned cost outside of Île-de-France "外省7-8万" but did not mention the cost to live in Île-de-France itself. 

Either the writer presumed the cost to live in Île-de-France is the same as in Paris, or he thought Paris is a province.