Is it 3.50 per serving?

no. i admit i dont know what the average price is, but $3.50 is really a starter price or a fairly basic fondant cake. most people dont want simple starter cakes.
costs can easily head up to $10 a slice when you start using more structure, more real chocolate decor, more gumpaste flowers or accents, gourmet flavours etc etc etc. its why pricing your own cakes can be such a challenge!
and then your top level cakers charge $yikes per serving! think how duff charges $1000 minimum - if you want a very basic duff cake, for 50, that is $20 a serving.
xx

There's another thread right now with a similar question:

http://www.cakecentral.com/cake-decorating-ftopicp-6619635.html#6619635

Maybe you'll find some answers there!

Since there's no "standard" cake, there's no "standard" pricing. You have to look at Your costs and compute Your pricing.