When building Stitch and Glue kayaks it is really nice to have the pre-cut panels complete with puzzle joints on the hull panels and for the strip built deck it would be nice to have all the forms. But you do pay a premium for that luxury.
Cost wise I would guess that you pay about 10 to 15% premium assuming that you do not already have some wood lying about you can cut forms and strips from, but do have access to all the nice power tools used to cut the pieces/parts- like table saw, band saw, stationary sander. planner, etc.
Either way you build your boat you will most likely enjoy the process of creating your own kayak.

John VB.