With your starter pack you're well set up to get going. The question that arises now, what contains the experimentor kit that you need which is missing in the starter pack. Once you have that list, look what the components are costing. Most likely, you will get them for similar prices if you get those individually. Adafruit isn't in the business to give away things for free so you're paying for what you get anyway. You might also write a nice mail to the gals and guys at Adafruit and ask, what would cost an upgrade-pack from starter to experimentor pack.
Korman