The deal is, scientists in Amsterdam have produced a "color X-ray" of Van Gogh's "Patch of grass" (1887) and found an earlier painting that the artist rejected. From the Reuters story:

"It is estimated that one third of Vincent van Gogh's early paintings have been painted on top of existing ones. Van Gogh literally recycled his own canvasses," scientist Joris Dik of the Delft University of Technology said.

Here is greater detail, including speculation that the rejected painting is tied to "The Potato Eaters."

Question: Why would Van Gogh "recycle"? Was canvas relatively expensive? Or was it driven by the artist's preferences? In Tyler Cowen's break out book, In Praise of Commercial Culture (1998), he writes on point (pp. 19-20):

Falling prices for materials have made the arts affordable to millions of enthusiasts and would-be professionals. In previous eras, even paper was costly, limiting the development of both writing and drawing skills to relatively well-off families. Vincent Van Gogh, an ascetic loner who ignored public taste, could not have managed his very poor lifestyle at an earlier time in history. His nonconformism was possible because technological progress had lowered the costs of paints and canvas and enabled him to persist as an artist.

Cowen's over arching theme is that the arts generally benefit from technological progress. French Impressionism, no less, would not have been possible without the invention of small lead tubes that allowed painters to take their studios outdoors, where the effects of different lighting were studied. But none of this directly answers why Van Gogh would recycle, much less why one-third of his canvases.

More questions:
Have paintings become larger over time as the price of canvas has fallen?
Does this lessen the relative value of "Patch of Grass"? Did Van Gogh hate the painting underneath sufficiently that he just wanted to cover it up? Or did he love the Patch of Grass so much it was worth covering up even the mysterious face?
Does an artist's economy of necessity lend itself to artistic innovation?
Does creative destruction describe artistic innovation? A recent favorite example of mine is Tom Nozkowski, as recently featured in W magazine.

If the canvas isn’t right, Nozkowski simply reworks it. “I don’t like tinkering. Whenever I go back to a painting, I try to open up the entire surface—you know, run a wash of color over it, or I’ll scrape it down, or I’ll rub it off with a rag—so that everything is back in play,” he says. “They can change pretty radically. I’ve always felt that probably the good stuff will keep coming back.” (To avoid that “Oh s---” sinking feeling that can arise from erasing something good, Nozkowski keeps paper handy to quickly re-create images worth saving before they fade from memory.) Traces of what came before are often left visible, like haunting memories or jumbled-up dreams. “It’s like character in somebody’s face,” Nozkowski says.... “I believe that what I’m doing is actually very close to our normal way of looking at and thinking about the world,” Nozkowski says before getting up to stir the roasted red pepper and white bean soup he’s cooked up for lunch. “We slowly build up a whole web of associations and meanings.”