kits are often more expensive than putting your own together- you are paying for the convienience- also, sometimes it is difficult to find all of the exact fabrics that a kit may contain. depending on the kit, sometimes you only need a small amount of a couple fabrics-and if you are purchasing fabrics individually you may have to purchase alot more than you need (not bad if stash building) but that does kind of increase your cost...
every once in a while i pick up a kit- generally if it is on sale for a really good price- is fabrics i really have no desire to collect- or are not available other than in the kit- or i'm picking it up for a gift. 98% of the time i found after getting a kit i really didn't like some (or all) of the fabrics & wound up exchanging them for my own anyway- so i really only bought the pattern...and stash fabric..