Hope I wasn't too discouraging!
If you do pursue it though, civil litigation is where the money is at. But this means you have to be part investigator, part paralegal, part lawyer. Like I said, stay away from the srveillance companies, look for law firm or SIU (speciaol investigatiosn unit) insurance positions. Large companies tend to offer better job seciurity and benefits but lowsey pay. Smaller companies tend to be the better positions with higher pay but have their own risks.
You will also never be rich in that profession but can make a good earning. New investigators cn make anwhere from $20K's-$30's. Experienced investigators are generally around $30K-$45K. Senior investigators about $40K-$60K+. Owners of a small to medium sized company can earn in the area of $300K, but that is a 24/7 job and not for everybody!
Good luck.