exhaust valve replacement cost? 1.8 zs

looking for an estimate of cost , mobile mechanic has diagnosed the rough running of my car down to a burnt out exhaust valve, i don't know if it's worth doing ,he says it will fail its mot on emissions . i have until november , but money is really tight at the moment. so not sure if i can afford it knowing i have some other bits needing doing for the mot. unfortunately its not a job i can do myself.

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