$2.00 - $3.50 per square inch depending on complexity of the painting. PLUS MATERIAL AND SHIPPING COSTS :)
For example painting on the left is an 11x14" complex painting as there is basically an entire scene with background nature, animals, and figures. The price for something like this would be around $420. The one on the right is 16x20" it is less complex as there is only one figure and minimal detail in the foreground and background the price for something like this is around $520.
It will usually take 2-3 weeks.
If you give me some ideas I'll do some mockups in photoshop, see if you like one, and go from there.