Price vs cost

[rhymepuma]

I'm setting up a Shopify on-line fine art gallery/store where I will be selling hand painted acrylic paintings on canvas and fine art prints.  In setting up cost and price levels for hand painted canvases of a range of sizes, varying amounts of supplies used, and time invested in creating the art, I need to know if setting these amounts is arbitrary or are there general rules?  It's difficult to figure any "exact" cost because materials vary in price and the time/labor involved in completing a painting also varies.  Any advice would be appreciated greatly.