Since it looks like you're basically buying the LadyAda kit [that circuit definitely looks like one of hers] and building it, I could see $30 being a decent price (as long as you're using the v3 of the kit) IF you're throwing in the batteries.
I can't see paying an extra $10 just for you building the kit though (sorry, but I know how easy it is to solder those).
Unless you can get those parts significantly cheaper than what LadyAda is selling her kits for, I don't see this being very profitable for you.
Newegg has the same basic thing in a (admittedly less cool looking case) for $20.
I've seen similar [not identical] items got for as low as $2 in the past: http://www.goldengadgets.com/emergen...e-charger.html