Installing a shower cubicle can be a replacement for your bath or a new addition to a bathroom. The cubicle itself will make up a large amount of the cost of the job, but that will depend on the size and model of shower cubicle and fittings that you decide upon.
Choosing something standard from a DIY outlet will certainly cost less than other brands, but the cost for fitting either will remain the same, as a skilled plumber should find the job fairly simple to complete. They should also be able to advise you on models to choose, sizes and fittings such as taps. This is where hiring a skilled professional comes into its own. Part of what you are paying for is their years of knowledge and experience, which can certainly save you money.
For a new shower cubicle you will probably need some pipework to be done by your plumber, so that the shower fits seamlessly into your water and waste pipe systems. If you choose to have an electric shower installed then you will need to speak to your prospective plumber to make sure they deal with such types, as not all plumbers do.
The cubicle itself, including shower tray and doors, will need to be fitted well by a professional in order to look good and to ensure there are no leaks when it is used. You may need to have someone in to redecorate or re-tile if the shower is being fitted into an existing bathroom, or to add additional tiles where the shower may make the wall wet.