If you're doing any kind of professional photography you should base your prices primarily on your cost of doing business. That means figure out your monthly overhead costs (film, processing, studio rent, equipment, insurance, assistants or other staff salaries, etc.). Then figure out how much you want to make per month as your salary. Add it all up, divide by the number of days per month you think you'll spend shooting and it'll become clear how much you need to make on those days. That should be your absolute minimum price. It's not worth working for less than it costs you to keep the doors open (even if those are metaphorical doors and you don't have a physical studio). You'd be better off spending those days promoting your business instead.
Even if it's just a sideline gig for you and you're not looking to start a serious business, it makes sense to do this basic business planning. You don't want to lose money and it's bad for the photo business when part-timers undercut other photographers who depend on photography to put food on the table. And, of course, you don't want to sell your own talent short.
After that you can do you market research.