i think you are probably selling yourself short.
the materials costs sound fine, doubling the expense is no problem.
in addition, i would keep the extra beads and findings. when i make
a custom piece, i certainly do not give the customer back any metal
filings or scrap! (just as an example)
however, i am concerned about your labor fee. how long did it take
you to make the necklace? including time spent ordering materials
and designing? $15 sounds cheap to me, as though you are
undervaluing your time. what do you charge per hour? what do you
think you deserve according to your abilities? even if all that
work took you one hour, $15/hr seems cheap to me.
i would invoice the gallery with one price, do not break it down.
something like "custom (whatever kind of bead) necklace: 180.00
(or whatever you decide to charge). your final charge should be
exactly what you would charge at retail for the necklace, and no
we often undervalue ourselves and our work, and i think that is a
big mistake. people who want custom or handmade work are willing to
pay for it, as they should, and they should pay full price!