You are buying diligent professional work and complete reporting.
You are not buying a guaranteed happy or hoped for result.
Examples of personal investigative success
Workman's Comp Fraud: I captured 6 hours video of bartender at a sports bar while drawing workman's comp from another bar for a trip & fall.
Out of state client had concern about spouse coming into town for training (or more). His wife was met at the airport by her boyfriend. I was able to send the client video of them checking into a hotel room and evidence of the overnight stay together.
An internet provider suspected a customer was re-selling the service. Investigation indicated the customer was erecting WIFI antennas around town. After renting a house for a day and having the re-seller install his antenna & service, the originating ISP was provided written and video evidence. The case was settled out of court prior to trial.
If you're planning to go to court, I should be working for your lawyer, so my work product is protected under your client/attorney privilege, and no one can obtain anything that hurts your case.
If you're wondering if a Private Investigator is right for your situation, send me an email or give me a call. I'm happy to discuss the matter and give you my opinion of whether it's worth pursuing and what it will cost. I do not take cases that have little likelihood of success.