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 for free 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; the Airport Tavern would be twelve minutes from arrivals and the route is suitable for pedestrians.
Update Jan 2020: there is now a free waiting zone by one of the car parks, accessible by passengers by a short (also free) bus ride from the terminal:
A free-of-charge space for taxis and public that are dropping off and collecting passengers.
The Waiting Zone area is located close to the entrance of our Silver Zone car park and clearly sign posted upon entering the Silver Zone area.
The Waiting Zone allows up to 1 hour free of charge that may be used for the waiting of passenger pick up, prior to collection from the Short Stay & Pick Up car park.
There is access to a frequent bus service to the Terminal building if passengers do wish to be collected or dropped off in this area.
The Waiting Zone has pedestrian signage to the bus stop location close by. From the terminal, passengers will be required to use bus stop 4 on the forecourt and board the bus for the Staff Transportation Hub.