A good standard way to set prices is the same way any business would to be competitive.

• Add up the cost of all materials you used in creating a painting.
• Then add up the number of hours you worked on the piece.
• Then come to a determination of how much your time is worth (something like $10-$50/hr or some other reasonable number).
• Then add the percentage of cost for your studio, electric, water, etc., that was used during the making of the artwork.
• Then add 10% for good measure (there are things which I am overlooking here, you'll have to decide what your costs are).

For instance:
materials: $25.00
hours at $20.00/hour: 6=$120.00
studio related costs: $75.00
subtotal: $220.00
10% added value: $22.00
price of a 6 hour painting: $240.00

It is important that your artwork prices always go up. So start low with the intention of allowing prices to rise. Professional galleries will normally ask for 50% of the selling price (sometimes more). This will raise the price of your painting to $480.00. Do not sell for less. Once the price has been set by the gallery. Raise your prices when selling direct because your gallery will want their cut.
Of course, if your works become more popular, you'll want to raise the price to what the market will bear.