You might try City-Pharma at 26 rue du Four in the 6th arrondissement (corner of the rue Bonaparte and the rue du Four, métro Mabillon or St-Germain-des-Près).

This is THE parapharmacie of Paris, offering a large selection of name-brand products at discount (often up to 30% off). They did carry Roger & Gallet products the last time I was in, but I didn't price them so I don't know what the discount may be. Otherwise, as wideman points out above, prices are pretty much the same no matter where you go unless a store is having an exceotional sale.