What will be your costs, Who is going to do the printing, How large will the prints be? If they expect you to provide a printed product you had better find out what these costs will be. Also how elaborate will these posters be, how much work will you have to put in? 1 hour, 2 hours, 3? Never put a flat rate on work like this, or you may find yourself. Doing and redoing a piece for a week or more for only $100 dollars. Unless your clients will accept anything you produce for them as final for $100 you need to scale your fees according to how much work it will take to get to the finished product. Personally I would start at $100 and work up from there. If you dont you may find yourself spending days finalizing a work for only $100 and end up making less than minimum wage. I'd charge $20.00 Dollars per hour with a minimum of $100. This way if you spend 6 hours on a work you get $120.00 etc. This also keeps your cliets from unessarily having you redo time and time again for no valid reason as they will be paying for the extra time. Just some things to consider.