I sometimes agonize over that question when presenting my art for sale to the public here on this web site. Some friends feel I price my art too low. The answer, of course, is that the value of art is what anybody is willing to pay at any particular point in time. Like the stock market, the price for art may go up or down, depending upon its availability and, of course, the popularity of the artist.
Size also can partly dictate price.
You might have been willing to pay more or less for any of those paintings. Or, you might not have been aware the paintings were for sale. When my paintings appear in exhibits, not everybody gets the news. Sometimes I charge less for paintings that appear in exhibits, because I like to see them sell. Eventually, I will probably establish a price for this painting, but it's currently hanging in my living room.