Is it the heating coil in the tank that works off of the central heating system is nackered or is it the immersion electric element.
The cost of an electric immersion element and replacement would normally be around €150.00 inc VAT . But note that if is due to corrosion it may be impossible to remove from the tank so the tank may have to be replaced also.
If it is the heating coil in the tank then the full tank will have to be replaced . Look at aprox €150.00 for the tank ( depending on size and type ) plus day for plumber to change same ( isolate system , drain down system, remove old tank.. fittings corroded etc., install new tank, modift pipework to match position of inlets on new tank, refill system, blead system inc rads etc + test system also dispose of old tank recycle)