Are you selling it as a site with the forum intact? Do you get much traffic? This would make a great name for fan site (The biggest Football Forum site in the world is cpfc.org - Happens to be my club too!) Potential is there, although i think forums can be difficult to monetise.
Wigan have limited support and before they got promoted were getting approx 8000 attendances so would be difficult to promote if there are other fan sites.
I guess to the right buyer you could be looking mid-low £xxx.
NB. I'm no expert this valuation is purely my opinion. Please take with a pinch of salt.