It's not worth it IMO. You may not technically have an A in the paper, but it's the overall UMS that makes up your final grade. One extra mark might push up the grade on that paper, but in terms of the final grade it likely won't make a difference. Maths isn't exactly a subjective subject either, most of the marks are simply "the answer is 6" or "used this formula". Unless the remark is free, I wouldn't bother paying for it.