I'd check out Etsy as well. The pricing will depend on the artist, size, and medium. I've seen customs ranging from $50 to several thousand dollars. The most important thing is to choose a price you are comfortable with and that you feel suits the size and quality of the painting. If it helps, here's a general idea of where the artist will be coming from when he sets his price: canvasses of a fair size range in price from around $20-$100; enough paint to cover that canvass is going to be at least $30 for oils, a little less for acrylic and a little less for watercolor; then there's terpentine, brushes, jars, etc. You can expect your artist to be putting $50-$150 worth of materials into it, and whatever else goes into the price is labor and name. As an Etsy artist myself, if I arranged a painting (oil) from a photograph I'd charge between $200-$600 depending on the size of the canvass.