I've got to emphasise the point made earlier about quick release mechanisms: if you can get your roof box on and off easily using levers versus nuts and bolts you are far more likely to take it off when you're not using it. That means you'd save far more in fuel than you'd save buying a cheaper one that's a real pain in the wotsit to keep taking on and off.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.