BC Bike Race is definitely the one if you want the most singletrack, but withthe current exchange rate is hugely expensive (add in flights and accomodation either side of the event and you’re not looking at much change out of £2k).
Cape Epic has limited singletrack, but I believe the organisation is second to none. It wins awards each year for the quality of its event organisation, not just in terms of MTB events but global sporting events. Event is about £600 but you’ll have to enter the lottery to get a place for 2010’s race.
Trans-Alp will work out the cheapest simply because travel costs are much less, but you do have the disadvantage of sleeping in scout huts and the like, and having to watch Fritz lube his backside first thing in the morning….