If it was 40% over, then it is very unlikely that the overage would be seen as reasonable: the contractor would be less likely to recover it if he sued you for it, or if you sued him to get the money back, you'd have a reasonable basis for winning--a court could easily see a discrepancy that large as being evidence of fraud (that is, that the contractor deliberately misstated or lied about costs to get you to sign up with him). While no case is ever certain--don't believe any lawyer who tells you that you are guaranteed to win--a 40% difference would support you taking action.