The MCAT will cost $335 (at least). A fee structure based on timing of registration for the MCAT can be found here. This includes details of other fees and costs for registration/cancellation/rescheduling. For a brief summary, please see below:
Currently, the last day that you can register for the MCAT is 10 days prior to the test day.
There are reschedule fees that become more expensive as you get nearer to the test day. There are some cancellation refunds that are given, but they become less closer to the exam date. Check out the pricing here.
ADVICE: Overall, if possible, it seems most beneficial to commit to a day to take the exam early on. Rescheduling fees will be cheaper if you do decide to reschedule, and you have the option to cancel the exam if needed (this is not a possibility for those that register later).