It varies. I think some of the cosmetic brands that are common here (e.g. Lancome, Clinique, etc.) tend to be about the same price here and there, with the exception of at duty-free shops.
I've found that many items that tend to be everyday, inexpensive items in Paris are often expensive and hard-to-find in the US. There is a brand of soaps that I love that I usually find for about $7 at home was available for about 2.50 euros in Paris. In general, many smaller brands of cosmetics tend to have better prices in Europe than here.
Check out Monoprix. It's a chain store that's kind of half department store and half food hall. Their cosmetics are good finds (that's where I bought those soaps I mentioned).
Check out the major department stores. I like Galerie Lafayette & Au Printemps. While many of the brands they sell are designers or are on the expensive side, I've also found that they also have house brands that tend to have very good quality merchandise for decent prices that you can't find at all in the US. Unique French products tend to be my favorite finds.
I don't know how much shopping you plan to do, but if you spend more than 175 euros in one day at one store (or in all of DLP), you can get most of your tax back. Prices in Europe have the tax already accounted for (unlike here, where you get a new total when you go to pay). You have to fill out a special form at the store- ask about the "detaxe," and they'll know what you're talking about. They may need to see your passport, so it can't hurt to have it with you when you go shopping.
You can get tax back on most, but not all, items. Food items don't count, and I'm not sure about children's clothes. I don't think books count either, but I might be wrong.
