Walk-up single from Holyhead to London or v/v is £127 for peak time (which appears to be trains arriving in London before 10am) and £75.80 otherwise. Off-peak return is £76.80. There are advance fares as low as £12 single on sale at the moment for travel tomorrow, although some of the timings are pretty grim.
Anyone under 26 should consider getting themselves a
16-25 Railcard (issued on the spot at any staffed station on production of proof of age and a passport photo, and filling in a form which the station has). It's valid for a year and its 34% discount will get you back the price of the card if you spend over £76.50.