Hotel prices in Cardiff have been expensive since the concept was introduced unless you book 18 months in advance.
Novotel Cardiff are asking for £235 per room per night for both Friday and Saturday nights.
I have booked 4 rooms at the Etap Hotel in Newport, cheap and cheerful at £35 per room per night! Return trains look to be around the £50 mark, I'm happy with £85 for 2 nights.
Newport seems easy enough to get to and from Cardiff on the train.