One of my brothers went to university in Cardiff and I don't think I visited the city without visiting The Thai House. So, it's a bit of an institution for us.
The food is great and the staff are friendly and the service impeccable. The restaurant is always busy, so make a reservation before you go. I'm not sure how expensive it is as I never paid when we went (someone else always picked up the bill - lucky me) but it probably errs on the side of expensive.