There are two kinds of appraisal that determine how much a painting will cost:

1) Value Appraisal: The actual costs incurred in making the painting inclusive of materials, studio rental, electricity, and such, multiplied by two to cover the artist's labor.

2) Critical Appraisal: Regardless of materials and other costs, if the painting is reviewed well, is considered a major work, and/or initiates a new art movement.

The humblest of the humble will of course always base their prices on value appraisal.