Estimated surface area 12"x12" with simple subject, such as a "still life" of fruit, around $500
Estimated 16"x20" with one subject such as a portrait. Also landscape paintings $1000
Estimated 24"x36" one difficult subject which could include hands, extreme facial expressions, detailed clothing or background. Also good range for landscape paintings $2000
Estimated 24"x36" one or two subjects with details. Complicated clothing, plants, animals, background. More difficult and detail oriented landscape painting $3000
Estimated 30"x40" $4000 and up depending on subject matter and difficulty.