There are two different pay scales to consider in the private investigation industry. There is a pay scale as an employee of a private investigation agency or being the owner of a private investigation agency. As n employee you can expect an hourly wage between 15 and 
22 dollars an hour depending upon your physical location of employment (where you live). With most private investigation agencies, the agency will typically either pay for your fuel or pay you per mile driven. Mileage reimbursement can range anywhere from 25 cents a mile to over 50 cents a mile. Most of these charges are passed on the the client. Some companies provide bonuses for good performance.
As an Agency Owner you can expect a rate of pay anywhere between 45 dollars an hour to over 100 dollars an hour. This rate will vary depending on your client, what you decide to charge and what area of investigation you specialize in.
You must take into consideration on both pay scales that the amount of work you receive is never guaranteed.
The Bureau of Labor Statistics indicates that between 2008 and 2018 there will be an increase in growth within this occupation of 22 percent or 10,000 jobs. This is a rate much faster then the average of other occupations. The rationale behind the growth is increased security concerns and increased litigation.
The Bureau also indicates that the median annual wages of salaried private detectives and investigator were $41,760 in May of 2008. The middle half earned between approximately $31,000 and $60,000. The lowest 10 percent earned approximately $24,000 and the highest 10 percent earned approximately $77,000. Please keep in mind much of this information varied depending on the employer, the specialization and geographic area.
*Private Investigator Tip*
Many investigators believe that the more specialized you are in a particular area of investigation, the more likely you will be viewed as an expert. Therefore you as an investigator will be much more desirable to your client or employer.