Helen, I know already that I would love to meet you and talk with you… That said, let me just say that you are mixing apples and oranges. One’s subjective judgment of just one work of art, can come out just as you have described. Yours is a subjective judgment of the items you recognize in the painting which, for the life of you it seems, you cannot fathom why valuable to place on a canvas or whatever, and frame it.
That’s what I was intimating. While it may sound hoity-toity to suggest, you really do want to know a bit more about the artist, and try to get some idea of what it’s like to be an incredible genius artist. In a world where so few things are simple - yet are passed off as simplistic and with something akin to “I could have thought/done/made that” please look a bit further and do read that book. And, others, too, if you have time, that are about the life of an artist, or the artworld in general. I do think Tomkins is one of the most accessible authors on art topics, so his works (and Merchants and Masterpieces is one of several authored by him) are perhaps good to start with.
There is so much more to that Hockney painting, than just the brushstrokes/composition/choice of subject. While it’s important to have a visceral reaction, such as you have described, to an artwork, it’s also incredibly important to realize how much brainpower and sheer talent goes into artwork.
It’s a far greyer world than you suggest. And you look a bit foolish, dear (hope-to-be) friend, saying things as baldly as that. While it may seem fun and be a bit of a tickle to describe things so glibly, it’s going to be so much more richly rewarding to you if you might dig a bit deeper and read more about Hockney or whatever paintings/art works you are mentioning. My suggestion: say those things you’ve stated, internally, keep them in your head (I do the same at first). Then, get to know more. It’s almost like those glib statements that I have and you have, are a huge overt out-and-out invitation (if not a demand, to my brain’s way of seeing it) to FIND OUT A LOT MORE before I speak or think any further on it. It’s like I say, okay, I know it’s a cow and a nude woman and whatnot… But I’m not going to do the Emperor’s Clothes thing with a serious art piece. I save the Emperor’s Clothes thing for clowns like… say, Trump. 
But then again, this topic is about how to price a painting and as Tim states, $80 mil is obscene. Yeah. Man, what I could do with $80 mil. But then again, what I could do with… $1 mil. I dunno about stating the sale of it for $80 mil as obscene. I just don’t but maybe Tim does. So, I’ll be quiet about that in favor of saying that the world is complex and just wondering who would pay that much… But only in the ‘wonderment’ sort of wondering way. If that makes sense. I am, after all, a big fan of having artists finally recognized for having value and I guess this is one way that recognition is visible… The best thing about it, is the visibility it gives an artist, David Hockney. There’s so much more to discuss, like why art is stuck so often in art galleries/museums and is not in the public (though Banksy’s work is priceless in the best sense, and isn’t stuck in art museums/galleries). And, so much to discuss about why artwork makes news when its sale price at auction reaches such heights. Why only now, when the subject matter and the artist, to my mind, have so many other fascinating aspects?