A look at Exeter Airport’s website http://www.exeter-airport.co.uk/site today shows only one airline flying from Manchester. That’s “FLYBE” (http://www.flybe.com/) They have a flight arriving at 09.25 for £14.24 plus taxes, although I’d say that was cutting it a bit fine. A return leaving at 16:00 costs £27.24 plus taxes. It comes to £92.98 return with the taxes. They have 2 flights each way per day, so consider coming down the night before. Virgin Trains go direct. There are some cheaper fares, but not at times that meet your needs.