When you first start selling, I suggest that after taking costs into account, you decide what is the minimum price you would be happy with.
If sales are quick, then increase the price a little. Continue to do this if your work goes quickly. People are not happy if they purchase one of your works and subsequently the cost for other work you produce decreases. However if prices go up, buyers are happy that they bought when they did.
I price according to size so there is no confusion in people’s minds; they don’t see two pieces equal size and different price and wonder ???
It also helps when painting commissions as the person knows what they are in for before committing. My commission pieces are about 25% cheaper as there are no gallery costs.