Private investigator for Adoption Records?

I have been searching for my father's birth family for a long time. I recently petitioned the courts in Tulsa County, OK for his records, since we were always told from family that's where he was born. We received 3 letters back over about a 3 month period stating they looked for the private adoption records and could find anything. My question is about the use of a private investigator. How much do they run, what's a good company, etc? I'm so worried about getting scammed out of a lot of money but really want to find out this information for my dad. Thanks!