Pay a reputable person who does it for a living (approx. €130 is the current going rate), get their opinion, and ask how they arrived at the valuation.

You have every right to disagree but good valuers are fairly on the money most times. Bear in mind though they price for today, not for what it might be in 2 years time so they won't go lower, the same as they wouldn't falsely value higher in the past.