I think $150—$200 is about what professionals charge for an hour or so, at least when getting senior pictures… But since you’re starting out, you may want to knock it down a bit, to make sure they’re satisfied. Proffessional is kind of a state of mind too, since the grass is always greener. Also, you definitely want to have a flat charge for the time and have them pay for the prints at cost, or slightly above if you’re going to edit and print the pictures for them. $100?