The short answer: $24. The fee for registering after the deadline is $24, which of course is added to the regular $47.00 fee; 24 plus 47 is 71, so the total you’ll need to pay is $71 (assuming of course that you don’t have a fee waiver).
If you register after the deadline for the SAT, then you’ll have to pay a “late fee,” which, as of this writing, is $24. If you have a fee-waiver, you’ll still have to pay the late-registration fee.
More information: After you go through the long and tedious registration process (College Board seems to try very hard to collect a lot of personal information from you, including your Social Security Number), you’ll finally arrive at a screen that will explain the charges to you. If you click the i button (the information button), you’ll see an explanation of the cost, which gives you the following information (I’m paraphrasing here; you will see different wording):
Explanation of the charges: The registration deadline for this administration of the SAT has passed. To register, you will need to pay a late registration fee. The regular registration fee is $47.00. The late fee is $24.00. The total fee to register for the SAT is $71.00.
Moral of the story? Don’t register late! You can visit here to see the official list of SAT test dates: http://sat.collegeboard.com/register/sat-dates