Here are a few pieces under $300 that I’ve come across online. Most of them are pretty small, though that’s hardly surprising. Just the cost of paint and a surface to paint on is fairly high, and especially if a third party is taking a hefty commission it would be hard to make any kind of decent return on a larger work at this price point.
$280: 9×13 Koen Lybaert here
$200: 9×9 Dyanna Dimick here
$293: 14×21 Richard Shipley here
$280: 12×12 Marion Jones here
$131: 16×20 Mo Tuncay here