I haven't brewed from a kit in a few years, but I remember they were maybe $5-10 cheaper than buying the ingredients separately for an equivalent recipe. I started moving away from the kits because it seemed like they rarely turned out how expected them to be. I made a kit ESB that was not bitter at all, a doppelbock that came off as too thin, etc. and I was never satisfied with them. Granted, if you've brewed from a certain kit before and know its shortfalls, you can always pick up more hops or specialty grains on the side and make a little tweak while still possibly staying at or under the cost of buying everything separately, but that won't help when trying a new kit.
I like the ability to tweak and personalize my beers. When I brewed from a kit, I felt like I was just heating up a frozen meal for dinner and passing it off as my own.