I think Boulton's Trafalgar medals in white metal sell for anywhere between £50 and £1,000 depending on condition. If it is badly pitted it may be towards the bottom end of the range.
They were issued unnamed, but some recipients had their medals engraved privately. In my view this adds to their interest, but it doesn't seem to necessarily add to the value!
Do you mean by
'MO**s' that the 3rd and 4th letters are illegible? I have checked for possible matching names such as Monks, Moses, etc but I can't find a man with first name William. It is not unusual to find a medal with a name not in the musters, and one possible explanation is that he might have entered the ship under a false name (for example if he had previously deserted - or intended to in the future!) but had the medal engraved with his real name. But unfortunately it makes him virtually impossible to trace! Is there also a ship's name engraved on the medal?