Greetings Gary:    
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:    
http://web.extension.illinois.edu/forestry/pdf/tfb-nres-202.pdf    
Treble Stumpage = three times the stumpage value (i.e., three times the value of a standing tree).