Will my Colonoscopy at Kaiser Permanente be billed as preventative or diagnostic?

It is my understanding that a preventative colonoscopy costs less than a diagnostic one per insurance rules. It is my understanding that colonoscopies are recommended for those people starting at age 50. I am 58 and never had one. I have IBS (irritable bowel syndrome) symptoms. I asked my doc about getting one during my last exam and he gave me a referral. I will probably make an appointment soon...but am concerned about cost. Anyone have any idea? thx.