It is really difficult to give a price for a job like this, as there can be a lot involved. It depends on weather your having the whole bathroom stripped out, old tiles taken off walls, flooring taken up, removing old bath. Are you just wanting bath removed, and replaced with a shower cubical, and walls made good in that area? or a complete wet room? To give some idea, a decent shower cubical ( can be anything from £300 /500, if an electric shower is to be fitted then around £100/150 for the shower. A shower fitted to your boiler can be more expensive, so can the labour. With this type of work there can be various fittings and fixtures required to complete the job that you wont see in the brochures. If I were you I would contact 3 or 4 local plumbers/ bathroom fitters to give you quotes, the more local to you the better, then if there's any problems to be put right after the installation, there's a better chance of getting things sorted out, a one man business is, in my mind a better option. Good luck.