Three common pricing methods
There is no correct way to come up with the value of a work of art. Artists really just need to identify a method that makes them feel comfortable and justifies their work to buyers.
As simple as it sounds, some artists have come up with price per square inch (or centimeter) for their paintings and charge strictly based off of that. Artists that use this method generally have a price per square inch between $0.25 and $1.50. To come up with this value, the artist combines their quality of work, the price of their materials, the value of their creativity, plus any other factors that seem necessary.
This method is nice because it allows artists to have a very straightforward and rational model for setting their prices. It is easy to grasp because of the conventional thought that as something increases in size, it costs more. This works well if the artist's portfolio contains work of a similar style and quality. However, since this method puts a heavy weight on the size of a painting it may result in a low price for a piece that took a lot of time and effort.
Some artists like to charge an hourly fee for their work similar to a consultant or contractor. It generally takes into account more than just the time spent with a paintbrush in hand. Time spent sketching is also important to capture and in the case of commissioning a piece, the time spent consulting with the buyer should be included as well. Prices per hour tend to fall somewhere between $20 and $150. This wide variation is based on the artists value of their time and effort. Then, it is common for them to simply add the cost of materials to the final price.
This method is great because it places the value on the time the artist puts into the piece. Regardless of size, if the piece was difficult or required the artist to work slowly, their effort gets recognized in the price. This works well for artists that view their painting as a 'job'. This may not be a good method if the artist works so slowly that the price would be astronomical. Similarly, some impressionist artists can create large, high quality pieces in very short periods of time. This pricing method wouldn't quite reward their skill and mastery of the medium.
A simpler way to come up with a price tag for art is to look at other paintings of the same size, quality, composition, and medium. This is subjective but it is a quick way to arrive at a price.
Unlike the previous two methods, this one rewards artists that paint really well, quickly. It is very unstructured though which may make some people feel uncomfortable. Also, since the appeal of art is subjective it may be difficult to claim a price for your original painting based on the price of someone else's original painting.