All depends on the boiler, some I have done the expansion vessel
(original part) cost over €100, the the boiler must be completely removed
from the wall to replace (I always allow 3 hours)
Maybe €250 or so.
But if the vessel is leaking and the option of capping it off and installing
a 'standard' expansion vessel in either your attic or hotpress is available
it may work out a good bit cheaper.