Money: Frills and spills

Collect to invest: John Windsor on whether to buy a Di

Thinking of bidding for one of Princess Di's old dresses? You must be mad, or at least prone to hysteria - the communal disease that grips audiences at celebrity cast-off sales.

Remember the Jackie Onassis sale at Sotheby's New York last spring? It fetched pounds 22.8m, nearly eight times' estimate. So thoroughly did people, and money, get carried away that Sotheby's, in a shrewd public relations move, agreed to release from contract buyers who had bid silly prices, then woken up screaming the morning after. And prices were silly: Jackie's fake pearls, with a pre-sale estimate of $500-$700, went for a dizzy $211,500.

Bidders may get steamed up as they compete against one another but auctioneers remain coldly calculating. They know the value of provenance - that is, the added value that lies not in the object itself but in its vendor - whether it is on Old Master that has been cared for by the same duke's family for seven generations or a pair of pink ballet slippers worn by Rudolf Nureyev: pounds 12,075 at Christie's, London 1995. As the hammer fell, sobs were heard from an unsuccessful woman bidder.

The fact is, the big auctioneers have never - well, almost never - had a celeb cast-off sale that has flopped. The Onassis, Nureyev, Elton John, Barbra Streisand, Frank Sinatra, Joan Sutherland and Michael Caine sales have all been sprinkled with stardust. So the publicity hype goes on. But post-sale, in the cold light of day, the stardust loses its glitter. Speculators beware: you are unlikely to profit from celeb sales, at least in this lifetime.

Only the Britt Ekland collection (Christie's South Kensington last December) - although it sold 92 per cent of its lots, raising pounds 89,631, more than double the published pre-sale estimate - failed to achieve the lift-off into hysteria that is the auctioneer's dream. Typical of the bidding: a modest pounds 632, just within pounds 600-pounds 800 estimate, for Ekland's Zandra Rhodes smock and trousers, a little short on glitter.

Well, how popular is Britt Ekland? How many remember her? That does matter to an auctioneer trying to warm up a saleroom full of hesitant first-time bidders. Come to that, how popular is a cast-off Royal among either Brits or the Americans who will be joining the bidding? Celeb sales are barometers of popularity. Fans demonstrate their devotion by throwing money. Is Princess Di's Celeb Quotient worth pounds 3m, pounds 1m, or pounds 10m, in exchange for 65 chic confections of fabric? And would the wives and mistresses of the American rich really feel like a million dollars in her second-hand, out-of-date kit at a charity ball - especially if the dress were one of Bruce Oldfield's asymmetrical early Eighties efforts?

Fortunately for Princess Di, auctioneers have ways of making you bid. Apart from cooping you up in conditions resembling a laboratory experiment on aggression in rats, they lace the bait with silly expectations in the form of ridiculously low pre-sale estimates, published in the auction catalogue.

Such as the estimate on those paste pearls in the Onassis sale: did the auctioneers really expect them to raise as little as $500? Of course not.

But they understand the greed and aggression that such "come-on" estimates provoke. A handful of greedy bidders, having failed to carry off the lot for a song, will angrily turn on one another, bidding the price through the roof.

Even smaller auctioneers are up to the silly estimates trick. In the sale of the late Peter Cushing's belongings at Phillips in July, the actor's famous Failsworth green herring-bone deerstalker hat, worn in the role of Sherlock Holmes, was estimated at a piddling pounds 30-pounds 50.

"We're not really sure how strong his following is," said a Phillips auctioneer, all innocence. On the night, Cushing fans flocked and the deerstalker fetched pounds 1,380.

The fact is, without a realistic upper estimate to hint at restraint, silly bidders can go on bidding like hungry rats compulsively pushing a lever. Even Old Master prices are cranked up this way - especially at country house sales, where gullible bargain hunters abound. A Van Dyck portrait with a copper-bottomed attribution but a silly estimate of pounds 4,000- pounds 6,000 fetched pounds 133,500 at a Sotheby's country house sale in Suffolk in June - more than double its true value.

So when Christie's publishes its catalogue of Princess Di's dresses with estimates not much higher than their price new - pounds 8,000-pounds 12,000 - you will know what you are expected to do. Think twice before you do it.

If you must collect Royal costumes of a more intimate nature, why not go for a nice pair of Queen Victoria's knickers? She used to give them to her ladies of the bedchamber as perks (Victorian celeb value).

With their authentic VR monogram and crown, they crop up from time to time at auction, fetching pounds 600 or so. Having met the six people who collect them, I can assure you that they bear no visible signs of hysteria.