Without dates and times it's impossible to quote exact fares but the cheapest option is possibly to use National Express coaches. For next weekend (Saturday out, Sunday back), for example, they're currently offering 'funfares' of �10 each way. The journey time is around 6� to 7 hours but it's much cheaper than the train. (�65.40 single, �66.40 saver return).
http://www.nationalexpress.com/save/funfares.c fm
However, Megabus might be cheaper, depending upon your dates and times. For my sample dates of out on 22/03 and back on 23/03, they're offering outbound fares varying between �3 and �15, with fares for the return journey between �10 and �16. Journey times are in the range 5� to 6� hours, so you might get there slightly quicker than you would with National Express:
http://www.megabus.co.uk/uk/
Chris