I don't know how experienced you are at cakes, but I'm just starting out and I charge $1.50 per slice and let them know I'm less expensive because I'm not as experienced. Usually this covers my costs, and gives me a little for my labor on the cake.
But of course, feel free to charge what you want. My Wilton Course III teacher has been doing them for 20+ years and only charges $1.75 per slice. It can vary depending on the region you are in.