It's difficult calculating the average pay of a forensic psychologist because many psychologists undertake forensic work on a part-time and in some cases unpaid basis. Many of those engaged in the practice of forensic psychology have an alternative income stream e.g., teaching, research etc.
However, forensic psychology pratice can be very lucrative. A forensic psychologist who signed the forensic psychology guestbook stated that as an "expert witness" for the courts, he was getting paid $350.00 dollars an hour, and sometimes more depending on the case.
According to the British Psychological Society, Forensic Psychology is a growth area within the UK job market. Pay rates start around 20,000 pounds ($39,000) if you are newly qualified, up to 60,000+ pounds ($119,000+) for senior psychologists.