The most economical way to get to the airport is to take the R10, R11 or L12 trains, which take more or less the same amount of time as the FlyToget, but run a bit less frequently (every ~20 mins as opposed to every 10 minutes).
If you are travelling to and from the airport within 24 hours, then you can buy a 24-hour all-zone ticket for 190 NOK, which gives you unlimited travel around all of Oslo and surrounds including to and from the airport.
On a day trip you would normally only travel around Oslo Zone 1, and a 24-hour ticket for Zone 1 would cost you 90 NOK; thus if you make two journeys to the airport each journey would only cost you 50kr as compared to 360kr by the "express" train.
I know this reply is only a few months old but I was wondering if the 24 hour pass could still be used to get to/from the Oslo airport in Gardermoen. Ruter's website wasn't cooperating when I tried to find out how much a ticket from the metro stop near my hotel to the airport would be.