You sometimes get cheap fares put on sale online right up to the day of travel, so keep checking. £52 pounds is the maximum you'll pay, so no point in booking early. If you really want to save a little bit, leave Stirling at 0625, arriving Aberdeen at 0844, leave Aberdeen at 2042 arriving Stirling 2243. Buy singles. Cost £41.20. Any money you save will end up at Weatherspoons though..
Or Megabus via Edinburgh for £32, at more civilised times...