The obvious bus to catch is the A1 'flyer'; as you've noticed this is expensive at £11 return or £7 one way (if you use the mticket app, otherwise add a pound). However, since the end of October there is now an A2 'airconnect'. This is just £4.50 one way, with no need to book (and thus no risk of misconnect). Services only run once every 30 minutes though, and will take longer than the A1 (which now uses Metrobus lanes to avoid traffic).

Less scrupulously, your driving friends could pick you up from the old terminal building / car rental offices about a five minute walk from the active terminal. Or you could hike a bit further out of the airport estate entirely to a nearby public road, although you'd need to check on street view whether there is continuous pavement.