That estimate is not so much based on the artistic qualities of the work as on the highly unusual depiction.
I can't really find any similar examples; so the below should be taken with a grain of salt.
To me, it looks to be a depiction of Adam and Eve before they were split up. Huh? Well, according to some more obscure traditions, Adam and Eve were created together as one, before being divided. This is apparently an effort by some Rabbi of old to explain the difference between the two Creation of Man stories at the start of the Bible (Genesis). Yes, there are two of these, and finding an explanation that would make both literally true is not easy (well, it wasn't easy in the Middle Ages, it has of course become even harder nowadays).
I don't know whether the sources I will present are any good, but they do identify the story I mean, so here goes. Judaism.about.com tells us in "What Was the Androgyne?" about "a creature that existed at the beginning of Creation. It was both male and female and had two faces." The illustration with it is similar to the one for sale (not in any specifics, but in general). The website goes on to cite the solution: "R. Samuel b. Nahmani said, “When the Holy One, blessed be He, created the first ‘adam, He created him with two faces, then split him and made him two backs – a back for each side.” (Genesis Rabbah 8:1)" The image here also shows two naked figures, male and female, attached at the back (it's a fragment of a Greek vase). This was the so-called "primal androgyne". They continue with "A similar discussion can be found in Leviticus Rabbah 14:1 where R. Levi states: “When man was created, he was created with two body-fronts, and He [God] sawed him in two, so that two backs resulted, one back for the male and another for the female.”"
The myth of the primal androgyn apparently originated somewhere in the 5th century, andd was well-known in Jewish and Christian Platonic circles in the Middle Ages. This is better exaplined in the book "Carnal Israel".
Depictions of androgynous people are not uncommon in the Middle Ages, but usually they are simply two-headed figures. The subject is common enough to be dealt with in studies, as evidenced by this event at the University of York from 2014. It would be interesting to hear their view on this.
Another possible explanation (which doesn't have to exclude the first one, paintings didn't always have only a single meaning) is that the painting is some sexual riddle. Well, having a nude man and woman is not really a riddle, but they both blow a horn, and the man has spurs as well. Of course, the expression "the beast with two backs" was already known in French in 1532 at the latest (according to Wikipedia). Obviously, the painting shows the reverse of the expression, but still I can't help but wonder whether it bears any relation to it.
Value? No idea, it seems to be a very rare image but is it really some obscure apocryphal / kaballistic / gnostic image, or just some fun?