David,
You're talking of Heathrow, of Luton as examples, this is Rhoose we're talking about and many of these people come out of the valleys for their once a year bucket & spade holiday.
You're right, it is £3.90 Cardiff to the airport yet only £2.40 Cardiff to Barry, it was the £2.40 (ish) fare that I was recalling, so perhaps £5.00 ain't so bad but it's realistically only suitable for people arriving/departing Cardiff by train or people that live nearby one of it's very limited stops, other than those they're likely to continue using alternative means of transport hence my remark regarding empty buses.
As I said you can't compare London to Rhoose but the train fare from Birmingham New Street to BHX is just £2.40 and from Coventry to BHX a mere £1.50, Luton to LTN is £4.00 and these are significantly more of airports than Rhoose is ever likely to be.
CWL does need an improved transport link to/from Cardiff central but three times each hour from crack of sparrows until the owls come out and at an inflated fare, well I think they're going for an overkill. Particularly during a winter compare the number of aeroplane seats out of the airport in one day to the number of bus seats arriving at the airport in that same day.
And I thought the UK lived in a day & age of being conscious about carbon emissions etc!