Don't forget to consider the less quantifiable. You must feel comfortable with this person- a PHD doesn't guarantee they'll be successful in tutoring you.
Some tutors offer a money back guarantee if test scores don't improve by as much as 200 points. Be sure you understand the guarantee. Most have requirements regarding practice tests, homework assignments, etc..
Some tutoring companies only hire tutors who themselves scored in the 90th percentile on the SAT. They require that their tutors take a sample SAT as part of the employment screening. Why not ask the question.
How do you charge for your services?
What is your policy regarding cancellation and make up sessions?
How long are the sessions and how often should we meet?