GA are very welcome at Cardiff. Whilst there is mandatory handling that isn't as frightening as it sounds as the Flying club handles GA and it's not the costs that you get if you went with one of the handling agents
Give them a call on 01446 710000, they handle all the light aircraft south side and can give you an idea on fees.
Before you go to Cardiff take a good read of the AIP, there are quite specific rules for entering VFR such as entering the zone not above 1500ft QNH, there is also a one way taxi system southside but the flying club should also be able to advise you of all of this.
It sounds a bit more complicated than it actually is just brief well before hand.