I've only recently started charging for tastings myself, but only do take-away. I give them a nice box with either 12 regular-sized cupcakes with filling, or 24 mini cupcakes with no filling. 3 or 4 flavours for $20. Covers the cost & time to bake & ice them.
I figure, it's your business so charge what you feel is worth your time & supplies!
Kvand, I like your idea of multiple fillings!