By far the cheapest way I found for Glasgow to Aberdeen was with Megabus Parks of Hamilton services, these are not the big blue Double Deck services. Having worked in Aberdeen for 18 months I would go up on a Monday morning from Glasgow Buchanan Street bus Station and come back down on a Friday. Staying in Aberdeen is expensive and it proves cheaper to go on Sparerooms.co.uk and get a Monday to Friday room. I would get the 3:40 Monday bus to Aberdeen arriving in Aberdeen around 650am. The bus would stop at Perth and Dundee. You would get a sleep on the bus. Mondays and a day when there was an event like a concert the previous night would be busier. Concerts because people would finish the concert then head to a club then goto the bus station around 3 ish. I found that even booking in advance of a week seemed cheaper that going up the previous night. So i'd recommend going with MegaBus Parks of Hamilton as the cheaper way.