When hiring a private investigator, you should hire an expert that takes your case only if he/she believes that they can help you. The investigator should be clear about (a) What the exact investigative service and plan is, (b) how long it is going to take, and (c) how much it is going to cost. The costs should never exceed the agreed upon amount.
Thomas G. Martin is a professional private investigator and a former Federal agent. Martin only takes cases where he believes they can help the client. With murder cold cases for example, the agency only takes about 1 out of 10 cases because evidence is either unavailable or destroyed.