How do I begin a letter about ...?

Imagine you work as an appraiser for an antiques dealer. One of your biggest clients recently approached you about an antique desk for which she paid $25,000.00. She has asked you to write a letter to her insurance company, confirming the desk is worth the $25,000.00 she paid. She wants to insure it for the full amount. However, after you examine the desk and consult your sources, you find this type of desk was selling for $25,000.00 a few years ago, but now it is selling for approximately $13,000.00. You first carefully consider the ramifications and ethics of the situation you find yourself in, and then you compose the letter to the insurance company.