Add up the total time you will spend on the thing, and multiply by the hourly wage you would like to be paid. Add for materials and other expenses. That is your fee. If they don't want to pay it, let them go somewhere else, and put that time into your real job instead. (HINT: Catering pays well - about $15 to $25 an hour - considering that it has virtually no requirements except that you don't look like a total scumbag.) I would do a "quick" digital head shot for a FRIEND for $60, maybe...with no printing, and no carrying of equipment other than what I could carry on my back and in my hands, and no more than two hours computer work.
When I think about these things, I consider $25 an hour to be an absolute minimum for simple work (e.g. a whole lot of products on a uniform white background, intended for Internet only), and $40 or $50 is really more like it for involved work. That includes lab time. (Even these are more like semi-pro rates that professional rates.) Otherwise, it is absolutely not worth it. I'd rather catch up on my sleep.
At any rate, the most important thing is making sure that the actual time and money you spend on the thing divided into what you make for it equals a decent hourly rate. You improve your hourly rate by making accurate estimates of your time, and then actually finishing within the estimate.