I was asked if I could make a full sheet cake that looked similar if not the same as a dummy cake like the picture posted. Would like a little input on what price I should charge. Is 2.50 to 3.50 a fair price per serving?

AI assume the full sheet would be a single tier?

Your price would depend on your cost of ingredients, labor, and per-order overhead as well as local market value, probably between $3-6/serving.