How much would you charge for a 12 inch round carrot cake with butter cream simple design i'm thinking $65?

That's less than $1/serving if you're going by the Wilton serving chart. I would charge at least $100...and that's cheap!....but I live in a small town. I'm sure others would charge even more.

I go by the wilton wedding chart, which says this size cake serves 56. Rounding to 50, at my $3/serving rate (which is publicly posted on my website) = $150.

It's not a $150 cake ... it's cake enough to feed over 50 people.

I like the rule of 3 to be the absolute lowest price that I will ever give to a client:

Cost of supplies and consumables x3

1/3 goes to cost
1/3 goes to necessary and unexpected expenses
(i.e., insurance, electricity, etc. AND the always possible "Dang it! The chocolate seized! mistakes)
1/3 goes to profit

At this rate, you just manage to squeak through, IMO.
Anything beyond eggs and flour simple needs to be cost increased to cover you hours of effort!

I'd go bankrupt if I used the "times 3" pricing system.

Tell me about it! That's why I say lowest; as in you're my cousin and this is a favor because I love you. I know that may sound mean, to even "make money" off of family, but I have to factor in my personal expenses for when I do a project for them (babysitter, travel costs, etc.). When you put in hours, often days, on a project for a loved one, I don't believe you should go in the whole!

I own a catering business and let's just say that those prices are vastly different! A lot more overhead for building, professional equipment, business fees/taxes. I can't afford me to do an event! But that's ok because there are just enough folks out there who can to enable us to pay for DS's school!