Unfair price

I recently turned away a Renault something people carrier, turbo diesel and it was smoking. I suggested that fanning the flames was the best way out. Anyway, she got her neighbour that runs a garage to look at it, he diagnosed worn out turbo and offered to fit a secondhand one and did so, it still smokes, is seriously down on power and he wants to be paid 800 quid for the whole job, which was 300 for the turbo and 500 labour.