You can attend graduate school full or part-time, so the cost to you may not be as high on an annual basis as your bachelor's degree. If you are working, find out whether your employer will pay for you to take classes.
Universities also offer a limited number of graduate teaching and research assistantships. These positions pay your tuition and a stipend in exchange for working 20 hours per week during the academic year assisting in teaching classes or on a research project. Loans and financial aid may also be available.