How much wood it cost to repair or replace?

I have a Worcester 24CDI combi boiler fitted 9years ago. The pressure gauge went up to 4 bar constantly and after releasing the pressure it soon rose again.The air outlet valve blew and has been leaking on and off ever since, I have had advise from a family friend who had the same problem with the same model boiler that it is probably the heat exchanger that needs replacing. Can you advise what it is likely to cost if this is the problem

Mark Smith
July 2007