many kits are quite a bit more expensive than buying yardage---you are paying for the convienience of having all of the correct amounts of fabrics chosen for you.
if you have problems choosing your fabrics- don't want to buy yardage- or just love a quilt as it is exactly on the package- you will pay for it- but it can solve all of those issues.
if you prefer to choose your own fabrics, want to build a stash- and love a pattern- but not necessarily the color way it is in you will save $$ by buying yardage....i've seen quilt kits that were over $200===bought the pattern- chose fabrics and made it for $50
but then i've bought kits that were exactly what someone wanted- that i could not put together any other way....you will pretty much always pay more for the convienience of a kit being put together .