I've read the opposite. I've read that art should be priced by the square inch. Actual time involved is important, of course, but the public should get used to the idea that , say, an 18 x 24 watercolor of yours is going to cost, say, 300 bucks.
If you can knock out a watercolor in an hour, why charge less? OTOH, you may have to eat your costs on more complicated pieces.
With that, increase your prices gradually as demand increases. That's what free enterprise is all about.
I don't take the time spent into consideration on a piece because I have found that sometimes I can make a very good painting in a short time and other times I can spend weeks on something and it looks like crap.
I do take the size of the canvas into consideration. What I have is a little computer program that I wrote in a database (MS Access) on my lap top that does a simple calculation. When I add a new painting to my inventory database I put in the size of the canvas and the subjective opinion of the quality of the painting (poor, fair, good, very good, superior). The little program instantly calculates the suggested price by figure the estimated cost of the canvas, paint and materials and then multiplies it by a factor that is dependent on the subjective quality. Poor - factor of 1, fair - factor of 1.5, good factor of 2, very good factor of 2.5, superior factor of 3. Haven't ever gotten to a superior yet. I can change the factors at any time I wish by just editing the Quality table. Now this is just the suggested price. The actual price I enter taking the suggested price into consideration.
This system is working great for me.
I depend greatly on the comments I get on my paintings to decide the quality.