To figure out whether that price works for you, you need to first calculate your costs. Add up your ingredient costs, labor costs, and overhead (rent, insurance, license fees, etc.) on a per-order basis, then add 20-30% for your profit margin.
I agree. Without knowing where you're located we can't even begin to fatham a guess. If you're in Alaska or Hawaii then that is way to low. If you're in a rural farm area (hours from any large city) then you're probably too high for what others are charging. But if you're not making a profit then you're definitely to low.
Is it the $4/person that is causing you concern? Or is it the $600 invoice you're going to give to the client?
I find that many are bothered by a large invoice, without considering its enough cake feed ONE HUNDRED FIFTY people. Its how math works so go with it! 
I agree with Indydeb. Sometimes we need to get over our own personal perception of sticker shock.
I still remember the day about 3 years ago that my DD and I were in a local bakery getting 5 dollars worth of cookies. The woman next to us was order a big order, the lady asked for a $750 deposit. What seemed like a ton of money for cake was nothing to them. Boy, have I learned since then.
Quote by @%username% on %date%
%body%