The most "expensive" part of Paris, will be around the Champs Elysees. There you have some very expensive hotels, and certainely all the most expensive shops of Paris (mainly fashion).
Hotels in general are certainely not cheap in Paris. Housing is also expensive.
Because of the dense subway system (RATP and RER), it is not so important in which area of town you are : you can easily reach other areas easily.
I am only visiting Paris from time to time, for business, so my knowledge is not very detailed.

LUC