Not as much as you'd think
Downpipe & decat £400
Intercooler £250
Remap £400
K&N air filter £40
Brakes £350
Pipework £400
I've done all the work (apart form the mapping) so no labour costs so has cost £1,840
That means the car stands me at £5,840 which for 300hp and 350lb-ft of torque, and the level of comfort it gives, plus all the toys I really can't grumble at. Also - it costs no more to run or maintain than your average ford focus (well, apart from fuel costs) parts etc are all dirt cheap. There's not much for 6k that offers the same as I get from this car (that i'm aware of)