Undercharging helps no one. Here is a simple way to set rates you can stand behind.
Add up your time, travel, equipment, editing, and the years you have spent learning your craft. Your rate is not just the hour on set, it is everything that makes that hour possible.
Decide your minimum before the conversation starts so you are not negotiating against yourself. It is fine to offer a lower rate for work you love, as long as it is a choice and not a habit.
List your services and let clients come to you.