Is that the bottom? Maurizio Cattelan's mischievous sculputre Mini-Me

Mini-Me, a 14in self-portrait sculpture by the Italian prankster-artist Maurizio Cattelan, is perched on a bookshelf, surrounded by hefty art reference books. Offered at London's most recent contemporary art sales, he looks down, eyes wide open, as if in alarm. "How far down has the market gone?" he might be asking. "Has it reached the bottom yet?"

Well, there is an answer of sorts. After a precipitous plunge in February, when London's contemporary art sales fell by 83 per cent from £250 million last year to £43 million this, the New York sales in May fell by 76 per cent. Then last week's sales in London fell by a slightly less traumatic 73 per cent, from £261 million last summer to £70 million. At this rate, they might recover to 50 per cent of the peak by early next year, which would be relatively healthy.

As with the impressionist and modern art markets, contemporary art sales suffered from fewer lots and fewer high-value works consigned for sale, but there was a sharper decline in price levels for the most celebrated artists. While Damien Hirsts were selling, it was often a struggle. When one of his spin paintings seemed to be attracting no bids at Sotheby's, the auctioneer looked pointedly at the New York trader and Hirst supporter Alberto Mugrabi, shaking his head in disapproval before Mugrabi finally capitulated, bidding £195,000, way below the £250,000 estimate, to buy it. At Christie's, a Hirst sculpture of a child holding a charity collecting box, which had sold two years ago in New York for $456,000, was sold for £91,250, or $158,000.

Richard Prince has been an emblem of the art boom. Much favoured by hedge-fund managers, his "nurse" paintings soared from less than $100,000 to as much as $6 million in the course of five years. The first "nurse" painting to appear since prices crashed last October was therefore closely watched. Estimated at half the level it might have been a year ago, Country Nurse sold after little competition for £1.7 million ($2.9 million) – down perhaps, but still a great return on the original price.

There were several works on offer by Jeff Koons, but his current Serpentine Gallery show was not enough to revive past price levels. At Christie's, a 7ft-tall, stainless-steel cut-out in the shape of a walrus sold for £361,000 – half the price another from the series made a year ago. A smaller stainless-steel sculpture, Mermaid Troll, sold for £241,500 compared to the £512,800 another from the edition had fetched in 2007.

The troll was sold at Phillips de Pury & Co, where Charles Saatchi, who constantly recycles his collection, was disposing of more than £1 million of art. One work by Anselm Reyle, a 39-year-old Berliner whose shimmering silver-foil creations had seen massive price increases during the boom, sold for less than half the price similar works had realised last year. Paintings by Zhang Xiaogang and Wang Guangyi, two of the hottest Chinese artists during the boom, were unsold.

However, there was an upside. A butterfly drawing from the Saatchi collection by the American Mark Grotjahn, whose work is on display at the Gagosian Gallery in London, fetched a record £145,000 for a work on paper by the artist, and a painting by Yue Minjun, which Saatchi acquired three years ago for £249,000, sold for just over £421,000.

The shake-out is by no means consistent. For Andy Warhol, a bellwether of the market, prices in general have halved, but three works from the esteemed Froelich collection in Germany managed to sell at estimates that might have been set last year. A sculpture by Alexander Calder set a record (in pounds at least) for an indoor sculpture by the artist, selling to London dealer Simon Theobald for £2.6 million. There were also high prices paid for some living artists – notably the Brazilian Beatriz Milhazes, the Korean Do-Ho Suh, the Portuguese Joana Vasconcelos, and, not least, the Scottish-born Peter Doig, whose Night Playground sold for £3 million, the second-highest price set for the artist.

And if you can detect a glimmer of a smile in Cattelan's Mini-Me, perhaps it's because its price has doubled in four years, selling for almost half a million pounds.