In many cases, private investigators charge an hourly fee that is based on the complexity of the case, the location and other factors. Some services may be available for a flat fee, such as a background or record check. Many private investigators will ask for an upfront retainer before they agree to take the case. You may also have to pay for out-of-pocket expenses that private investigators expend to complete the service, such as mileage, airline charges, copying charges, long distance telephone call charges and photo printing charges.