What’s the going rate for a masterpiece? As Sotheby’s, Christie’s and Phillips kick off a week of spring auctions Monday, the market for trophy art is in flux.

The historic sale of a $450 million Leonardo da Vinci painting last fall continues to shake up art values at the marketplace’s high end, as auctioneers seek to manage collectors’ expectations and recalibrate price levels for dozens of other blue-chip artists.

Until...