The Economist explains why one of the Lenz lots–which was originally three–was so valuable:

One historic lot was a rare set of three paintings by Roman Opalka, who has conceived his oeuvre since 1965 as a single continuous work in which he paints white numbers in order from one to infinity on consecutive, mostly grey canvases; each new painting starts the numbers where he last left off (see slideshow above). The “1-∞” paintings were initially offered as three separate lots, but the artist intervened after hearing about the sale and requested they be sold as a triptych. The buyer of the Opalkas told The Economist that the “marriage value” of the three works was worth the £713,250 he paid. “Opalka is one of the great artists of the conceptual era,” he said. “Three consecutive paintings tell the story of the incredible beauty that comes out of this intense labour.”

Out from the Ashes (Economist)