As ben05 suggests, the majority of cheap fares have probably already gone. The cheapest route will almost certainly be via London, and you might find it also involves going to Manchester first and then coming back down to Wigan.
I've just had a look and you can get a single to Manchester from Euston at 09.20 for £26, and the 20.15 from Wigan direct back to Euston is also £26 single. You then get an off-peak day single from Manchester to Wigan on the day for about £4, so it's not as horrendous as I thought it might be by now.