Sophia Gardens is one of the better choices. Cars are £7 but not sure if a minibus has to pay a £15 charge as a coach. It's a pay and display machine and I don't think it gives change.
http://www.cardiff.gov.uk/ObjView.asp?Object_ID=8582
Take the A48M into Cardiff (left off the M4 after Tredegar Park), past the University Hospital (watch for the 40mph camera), go past the 24 hour Tesco and just as you go past the college take the left slip road. Left at the lights and stay on the same road, following it round to the right and then straight on for half a mile or so. The entrance to the car park is on the left. You will be quite close to the stadium.