I’d have a look at some of the recent Cycle to Work/Cyclescheme threads on here and work out whether or not you would actually make much of a saving.
As Al says, the Fat-specific components (wheels, tyres, cranks) are all pretty expensive at the moment. That’s why the Pugsley and Mukluk are actually really good value as a full build.
It’ll be interesting to see what sort of price the On One appears at – and what sacrifices (if any) have had to be made to get to that £1k price point.
Edit: That one on ebay is a steal. I’m guessing that reserve has been set at £750.