The most popular method seems to be charging per square inch. And beginners should start low. So $2 or less. My recent painting at 8x6 inches, is worth $96, which I think is reasonable. The issue here is that beginner doesn’t mean “I have no experience”. I personally think people should definitely practice and develope some skills before just selling. I also don’t think every painting should be sold. We should have paintings done for practice and paintings done for selling.