I usually price things like that more with an hourly rate that covers my travel, lighting setup, etc. This is because a person may get called into a meeting and you have to wait, etc.
You could do a hybrid, $X per hour and $Y per delivered image file.
I think if you get too complicated above that, or start to figure out pricing for usage rights, it may not pay off due to lowered referrals later. Outline the costs/rates so they understand, get their buy in and deliver what you say you will along with a detailed invoice, and this shouldn't be too difficult.
I have done this very thing in the past (a long time ago), but also follow the same model when taking pictures of entrees for a restaurant. Basically I set it up that they are hiring me as a paid resource with a skill (ie. photography) per hour, and for as long as they keep me around waiting, etc I am being paid.