i've also been down to cardiff on the train 5-6 hours we changed at sheffield and bristol on the way down and birmingham and sheffield on the way back , no problems . as for prices depends how early you book .
if you go on east coasts website you can set up an alert to let you know when tickets go on sale for the day you want to travel if it's more than 3 months in advance and you can ususly get a good deal

a word of warning to any one travelling on the train and coming back on the sunday after the derby,we was booking hotel and last train home from cardiff central the last train home to hull was supposed to leave at 6-45pm great no problems untill we checked when booking,and found out this had been brought forward to 5-45pm so no good if yer want to watch the full game the later train after this was 6-45pm stopping at birmingham then sheffield the hull connection leaves sheffield at 5-30am monday so all beware

Think we got the earlier train and it was a fooking nightmare, especially as the coppers wouldn't let us back into Cardiff Station after the game.

We ended up at Birmingham New Street before we got back to Donny.
There were no more trains back to Hull so they had to put taxis on for us.

Thank christ we'd won, otherwise I'd have been under one of them trains.

If you go to trainline.co.uk you can look and see what trains are on. If you want to go Sunday you either have to go saturday night and have a stop off over night in manchester or leave at 9 in sunday morning and not get to cardiff until 2:45 pm.

Not too sure about the train back.....

I was going to go but because of the trains it doesnt seem possible. The RFL did have a ticket and train offer going but they had to cancel that because the train company backed out.