You can pay a great deal if you buy a top-of-range Goretex or similar. If you're buying new then Altura are usually much more reasonably priced. Tchibo often have waterproof jackets of some sort in which are well-specced for the price. It's sometimes possible to get a cheap PU coated nylon mac (the sort that makes you sweat) from Chamberlains in Camden. You could also try Halfords or the bargain chain stores (Oxford Street) such as Sports Soccer or JJB.
To get an idea of prices look at the prices on-line.
Charity shops usually have second-hand rainwear but not cycle-specific stuff.