It depends on the kit as stated above. The True Brew Red Ale Kit for example cost about $33. It has 2 cans of extract that retail at about $26 total.
1 oz of hops at $3.50
Specialty grain at $1.50
Grain bag at $.25
Yeast at $.50
Bottling sugar $.75
Bottlecaps at $1.00.
So basically its a break-even proposition. For someone first starting out, its a value to have it all ready there in the box. Not to mention you learn about the ingredients and can begin to critique or think about what you'd change about it, add to it, etc etc.
For others that are at a point in their brewing that can create their own, by all means save that dollar and possibly make a better beer.
-J
