It mostly depends on the size and medium of the finished, non-commissioned artwork. With only very very rare exception, I don’t otherwise alter the price based on content or difficulty of execution. Unless a lot of 23K gold leaf was used (for example), it’s just not worth it to me to try to justify to a potential customer why this 40 x 50 cm (16 x 20 in) piece costs more than that 40 x 50 cm piece. They almost always go for the lower priced one in any case.

As of January 2019, prices range from $175 for a 5 x 7 inch acrylic painting to $8000 for a 36 x 48 inch acrylic painting (larger than that is negotiable). Prices increase more-or-less annually, depending on market conditions.