When we come over for the TT we're there for 18 days and I figure we EACH spend around 1400-1700 pounds while on the Island (I tend to buy more souvenirs like signed commemorative helmets and books).  That includes lodging, car rental/insurance, petrol, food and drink, event/facility tickets and souvenirs.  That works out to between 77.78-94.44 pounds per day and I think that's reasonable for 18 days for an event like this.  It doesn't include plane fare to and from (add another 750 pounds for plane fare).