If you fail to meet the audit requirements, PMI will refund your exam fee, minus $100 administration fee for the audit. Now you know why they ask you to pay first :)
However, PMI has clearly mentioned in the PMP Handbook that they reserve the right to audit any candidate any time i.e. even after they achieve the PMP credentials. If you fail to meet the audit requirements after you achieve your PMP certification, you will not be eligible for any refund.
Note: I'm still trying to find out how and when, if at all, a candidate is eligible to re-apply for PMP certification once he/she fails the audit requirements.