Actually, that dinner for 25 on 1 day's notice is more the province of the private chef -- who works for only one client -- than the personal chef, who usually has several clients for whom s/he typically provides a week's worth of dinners at a time.
There is much more to the business of being a personal chef than just cooking: insurance, transportation, marketing, accounting, among other skills. I'm familiar with Candy Wallace of
American Personal & Private Chef Association and think she's very good at training the business aspects of the job.
As damack says, the cost of training is an investment -- and ends up being tax deductible.