I have gotten lots of emails asking that question. It's difficult to answer because costs vary by location (metro or rural areas) and by photographer (professional, chain studio, weekend 'digital Debbie').
Most HIgh School Seniors spend a total national average of $1200 for Senior Portraits. That figure was provided by the Senior Portrait Photographers Guild in July of 2008. That figure includes total costs. Retouching, Location fees, Prints, Frames, Graduation Stationery, and Shipping.
Many folks like to say "ouch!" when I tell them the figure... but let's be honest! I always reply with the question "How much did you spend on Clothes alone for back-to-school wear?" Your student's Senior Pictures will remain a longer lasting representation of this time in their life much, much longer than the Hollister T-Shirts you bought them or the Lucky Brand jeans they wore for half a semester.