You must have Masters’ before earning PhD., either from another school or the same school. It usually requires 4-6 years of graduate school for an individual to complete his or her thesis (research project). At many schools you do NOT pay tuition. At many schools you receive a stipend, a form of salary for serving as a Teaching Assistant (TA) or Research Assistant (RA). A PhD offers greater job opportunities and greater salary than a Bachelor’s or Master’s degree. In addition, a PhD allows you to be licensed by a state to do certain activities (e.g., testing, therapy). Read about comparisons between Masters and PhD degrees in psychology here.
abouteducation.com lists the average yearly income for an Experimental Psychologist at $56,000.