Probably double that in the case of shakey_hand_man.
Just a forced induction conversion is one thing, making comparable mods to your car to make to worth it and liveable with could cost as much as engine mods. Think good coilovers, 500 upgrade suspension parts and an LSD for starters at around £2k. £400 for great tyres.
I think Andy's (shakey_hand_man) car is great and his mods are high quality and in good taste. His car is like standard to live with theoretically, so little compromise there. However, it clearly wasn't so much of a hoot to drive since he had bought a Clio with renowned good handling within weeks of getting it reliable. Wheelspin in 3 gears and worse handling than a standard 1.4 500 with a lot more torque from the conversion? Fun, but within limits.
BTW, Andy's car is a bargain if you want to force induct a 100HP. It demonstrates how difficult it is to sell a modified car perfectly. It is a nice low-mileage, late model example with really good, tasteful mods that cost loads more than he spent, yet it doesn't sell. You could return it standard and sell the modified parts and make a profit if you have time.