Fuel is easy to calculate, as is insurance and tax (as you've already done). The bigger costs over the ST will be things like tyres (£500 a set), brakes, clutches and any other consumables really. There's also the fact that the essentials cost a lot more: If you grenade the engine or 'box on a Fiesta ST it's not going to cost you as much to replace as on a 350Z... That's very worst case scenario though, and is incredibly rare on this car. Always worth keeping in mind though.