As indicated below, FlyBe is the only air carrier to Benbecula at eye-watering prices (£300+ return unless you can book very early). They maintain these even if the flight is half full.
Beware - security at Benbecula is unsmiling, excessive and extremely tedious. You may be selected to have your private possessions picked over in full public view.
Caledonian MacBrayne, on the other hand, has no security - but it takes all day to get there from Glasgow (via Oban).