It really depends on a lot of things. What are you covering the cake in? Buttercream? Fondant? Ganache? Are you filling it with something yummy like raspberry filling or just buttercream? For a cake filled with buttercream and frosted in buttercream I charge $2.00 a slice. For Fondant it's $3.00 a slice. Want a filling othe than buttercream? Add an extra $0.25-$0.75 a slice. A 6 inch cake will serve 14 wedding slices (1"x2"x4" slices) and a 10 inch will serve 34 wedding slices. So that's 48 servings.. I'd start at $96.00 and go up from there depending on options. HTH's 
I think that sounds about right too. My 10" runs about $35 and then a 6" is about $20. Stacking cakes is a little more costly with a few extra supplies, not to mention the time that goes into it. So, for me I would charge, for just basic ingredients, $55.
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