Ok everyone... a little help needed. Say you are making just a basic sheet cake. What would you charge for each size? 1/4 sheet? 1/2 sheet? full sheet? I think that I am undercharging. Also, everyone's opinion on servings seems to be different, so if you don't mind putting in your own opinion about how many servings per each sheet cake size I would appreciate it.

Many people charge by the serving. I charge basic price $2/serving for sheet cakes.
Sheet cakes are a single layer of cake, iced/decorated, but not torted/filled.
Filling should cost more per serving, because it's more work and you use more supplies.

If you make a layer cake by stacking 2 sheet cakes, then servings become taller and narrower and the number of servings per cake is doubled. These are often called "kitchen cakes" at a wedding, for more servings beyond the stacked/tiered wedding cake. They are iced/filled/decorated similarly to the wedding cake so as to be undetectable by the guest as something different from the wedding cake. Charge accordingly for layer cakes. Twice the servings = twice the $$$

In my area the following is standard:

9x13 aka Quarter sheet = 24 (2x2x2) servings OR 2-Layers = 48 (1x2x4) servings

12x18 aka Half sheet = 54 (2x2x2) servings OR 2-Layers = 108 (1x2x4) servings

18x24 aka Full sheet = 108 (2x2x2) servings OR 2 Layers = 216 (1x2x4) servings

Every place is different as to how much you will be able to charge. Call the bakeries in your area and ask what they charge. Pretend to be getting prices for a party, if it makes you feel less embarrassed to ask. Don't undercharge for your work. Account for everything when you decide how much to charge.