After one year for a general rule it’s 50% of new price, for every year after that i generally knock off another 5% if in very good condition or 10% if in poor condition.
So if it was £3000, new 3 years ago, bearing in mind 2014 is round the corner and the bike is in good condition with nothing needing replaced i’d say £1350, maybe pushing £1400 max, if in a well used condition i’d say £1150 to £1200.
However there are a lot of idiots on ebay who get bid fever and pay daft money for stuff so take a chance.