Department store cosmetics are worth the cost because they are more fresh. Drug store cosmetics are often on the shelves for months and may flake or give you an allergic reaction.
It really depends on the brands you are buying. Companies like Almay produce good quality items at a lower price. Mascara is one item I don't buy at the drugstore.
Department store cosmetics are worth the money due to the ingredients used for the products which are specially formulated with advance research and development.
Typically, department store cosmetics are made from better ingredients, and they tend to last longer. However, there are many drugstores that carry high-quality and still reasonably priced cosmetics.