cost of new expansion vessel?

i have a vaillant 828/2e condensing boiler and i have to keep topping up the water level of the boiler too much in my opinion and it has been suggested i am leaking water in the heating system radiators etc but there is another solution i have been told the expansion vessel diaphram may be perforated.If this is the case how much is the cost of a new expansion vessel.

hope somebody can help me regards john nolan