£4000 is very pricey, but, that will be for Jenvey ITB's which are proper throttle bodies. Bike ones work just aswell but will be a little bit more hassle. For a bike tb'd setup i'd set aside £100 for the tb's, £150 max for manifold from Bogg Brothers, £50 for silicone couplers, unless you bolt them on. Then ideally you would run standalone management like my old MX5 which was on Megasquirt. Management systems that I know work well with Pug/Citroens' are Megasquirt and Omex. Good systems DON'T come cheap, you really do get what you pay for. My moto is 'Buy cheap, buy twice!' Then you'll need it mapping for your setup, again doesnt come cheap and you really need to find a reputable company who have good experience with the management system you chose. I'd budget around £1500-2000 for a 'd.i.y' setup