This is actually quite complicated even though the underlying formula is rather simple. in short:
Consider your time and assign a value.
You will have to decide what you want to make per hour. We feel a fair rate of art time should vary from $15-$100 depending on image complexity and your skill level.
Track your materials and cost of production and mark it up (or other intangibles).
The justification for marking up your materials is to both recover your expenses on a project and be able to buy more to keep producing. A typical markup on materials is between 15%-50%. Some expenses some people forget to factor into the formula is gas, phone calls, electricity, paints, brushes, travel expenses, and many others. You really just need to think about everything involved in bringing an image to the world.
Supply and demand
There is one more thing to remember, if you have any image that sells better than others the price may need to be adjusted up.
I made an excel spreadsheet that simplifies this process :
If you have any trouble with it below are the manual formulas:
1) Original painting price (what you need to make)=Time+ marked up expenses
2) Retail Pricing for a Gallery = (100 multiplied by your price) divided by (100 minus gallery commission)
I know it looks confusing but believe me it works every time. What it accounts for, which can be tough to accurately figure, is a gallery commission. this formula (or the spreadsheet) will make sure you hit your mark to the penny every time. This is a new calculator so please let me know if you have any input or discover any bugs. Thanks and I hope this helps.