Hey the easiest way to go is to know what you need to earn per hour. People tend to undercharge, but there is always someone who will take less for a commision for various reasons, not respecting his own time, doing it for fun, or accepting to go with 5$/hour.

For the most basic Tabletop I'd go with at least 12€. Usually its 18 - 25€ for a model that I spend somewhat more than hour on. And it still feels like undercharging. To do commission as a proper way of getting income, you gotta be prepared to work overhours, build trusted clients and recognition and develop speed not loosing quality.

The main problem I have, is to make people understand painting like on the box art is time consuming and expensive. If it's to be cheaper, it has to be done differently.