Professional foresters are frequently consulted to assist private and public forest landowners with timber theft and timber trespass issues.
Therefore, I strongly encourage you to hire a professional consulting forester to assist you in determining the value of the illegally harvested timber and to help you with other mitigating factors (property damage, tops/logging slash, damage to pre-merchantable stems, etc.).
Directory of Professional Consulting Foresters:
Treble Stumpage = three times the stumpage value (i.e., three times the value of a standing tree).