I've spent time and specced my own supercharger build last year. To do it properly you'd be looking at 3.5k buying new parts and good quality stuff including mapping etc. Good for just below 300bhp on the standard engine. I wouldn't go that far my self, I was only going to aim for 250/260bhp. In the end I decided I didn't want to spend that money on the Clio i've got at the moment. Already in the region of 12k after purchasing the car a few years ago total spend and should probably grow up and start putting money towards a mortgage at some point and keep some money in the bank for if anything every breaks down on the car or my van.