PMI members enjoy $150 discount on the PMP exam fee. However, you must be a PMI member at the time of submitting your payment for the PMP exam, in order to enjoy the discount. If you sign-up for the PMI Membership after submitting your payment for the exam, PMI would NOT refund the membership discount amount to you.
Therefore, it is important that you sign-up for PMI membership before you submit your payment for the PMP exam. For more information, read 10 Benefits of PMI Membership.