So many other things it could be as well. Possibly the circuit board, or the hot water thermostat, or any of the sensors and probes that, for safety's sake, may be preventing the HW from working.
With the boiler still providing the heating ok, then I wouldn't give up quite yet.
Replacement will vary depending on boiler size, type of flue, size of gas supply pipe to the boiler.
Allow at least £2,500 for gas, and maybe up to £3,500 for oil.
Just guesstimates though ............... Ankou must have slept with the plumber for that price ;o)