£5500-£5700-ish for a 2013-2014 bike. The difference only being plastics colours, the engine and cycle parts are identical.
Registration takes a little bit of doing as there is no CoC available as they are not EU type aproved. But there is a loophole in the DVLA system to allow motorcycles for competition to be registered.
They do not have brake lights, speedo, horn, hi/lo beam so a little bit of work is needed to make them road legal, but all the kit is available a faily cheap price.