There was a case in the UK many years back of a woman who had been to India with her husband. She had bought a ring when she declared to be valued at $100. Later Customs found out it was worth $100,000. They took her to court and the judge decided that she was entitled to declare the cost as anything and the Customs had the right to call in an expert to get a true evaluation if they wished to. They had not done this so the case was dismissed.
It probably isn't the same here in Thailand but the Customs do, I would think, have a responsibility to check the value of an article is they don't believe the importer is telling the truth.