You've seen the art, now buy the hat

If you go to the Royal Academy, you can buy Caravaggio's hat. It is a rather wonderful hat. The exhibition currently showing at the RA in London's Piccadilly is called "The Genius of Rome", and has paintings by Caravaggio and Rubens and others of that ilk, some of them never seen in this country before. The paintings are not for sale. But the hat is depicted in Caravaggio's painting The Cardsharps of 1595. And you can buy it. It costs a mere £1,500, give or take some VAT.

If you go to the Royal Academy, you can buy Caravaggio's hat. It is a rather wonderful hat. The exhibition currently showing at the RA in London's Piccadilly is called "The Genius of Rome", and has paintings by Caravaggio and Rubens and others of that ilk, some of them never seen in this country before. The paintings are not for sale. But the hat is depicted in Caravaggio's painting The Cardsharps of 1595. And you can buy it. It costs a mere £1,500, give or take some VAT.

Caravaggio's painting is all sensuous, sumptuous cloaks and clothing in opulent velvets. The hat - or one very like it - is worn by one of the players, who appears to be cheating. The hat you can buy is, of course, not ripped out of the painting itself. It is a "school of" Caravaggio hat, an "inspired by", "in the style of", an exclusive recreation of a hat, made by Pip Hackett, who for those in the know is one of Britain's top milliners. It is on show in the exhibition shop. There is, not unnaturally at that price, only one, so hurry while stocks last.

Other goodies for sale include a baroque sculpted Cupid necklace - a snip at £1,200, even if the glittery bits are mirror and beads and semi-precious stones rather than diamonds. That, too, is a one-off. But you can order the waistcoats in velvet silk brocades with hand embroidery. Tom Gilbey has made them, again inspired by the paintings, and they retail at rather less than the hat - a mere £395 apiece.

You can buy, for £75 a shot, the pewter plates and jugs from his paintings. You could, given a healthy budget, throw your own Caravaggio dinner party, buy into Caravaggio, live Caravaggio. It is all a far cry from the usual museum-shop offerings. Once upon a time the tables by the exit sold postcards and table-mats with cheap reproductions on them, and that was about it.

"We still do the postcards and prints; but this did seem an opportunity and a good idea," says John Barford, the RA's general manager of retail. "We approached three top designers, showed them what we were going to hang, and asked if they would like to do exclusive interpretations in their own media for us." Hackett saw the Caravaggios, she was commissioned, she created. "We haven't offered anything this expensive - not over the thousand-pound mark before, at least not in my time, which stretches back 10 years," says Barford. "But we may well do so again. We are putting on a Botticelli and Dante's Seven Circles of Hell exhibition in the summer - that should provide plenty for the imagination ..."

The incentive to let imagination rip is really rather clear: Royal Academy Enterprises Ltd, the RA's trading arm, had a turnover of £7m last year, with a profit of £1.5m, all of which went to the Royal Academy.

The RA shop is not alone. In fact compared to the British Museum it is trailing. There you can buy a plaster cast of a horse from the Parthenon frieze for £625; a bigger section of the Parthenon, showing cattle and cattle-herders, for £1,400; with, best of all, a cast of the statue of Nike - life-size, from the Temple of the Nereids at Xanthos in south-west Turkey - a snip at a mere £4,500.

Considerably less classical, but striving towards the same price range, is a board-game called Outrage! available from the Tower of London. The idea of the game is to steal the Crown Jewels. The standard set comes in around £30; but anyone, surely, of taste and discretion - and wealth - would go for the de-luxe version, in which you play for real jewels. It retails at £3,995. They last sold one two years ago. But even that is not the Tower's most expensive gift. A Royal Worcester figure of Henry VIII, one of a limited edition of 75, is on offer at £8,105.

Compared to that, Tate Modern is way behind. They opened last week their "Century City" show; and they have completely failed to capitalise on reproductions of Freud's couch, as displayed in the "Vienna 1908-1918" section; the best the exhibition shop can offer is its very hefty catalogue, at a slightly-hefty £25.

The Science Museum has been into such things for some time. Hence the Weatherstation. This is no tin-can, rain-gauge and mercury thermometer, but everything all-singing, all-dancing and digitised, offering state-of-the-art read-outs from as many electronically-monitored sites around your house and garden as you fancy. It costs £549.99. The Science Museum has just opened a retail outlet in Selfridges - so for a museum shop, you don't even need a museum any more.

And museums need shops. The shop and marketing company at the National Railway Museum in York makes a profit of £120,000 a year, which goes straight to the museum. It, too, is experimenting beyond the souvenir key-ring, with a pewter model of the record-breaking Mallard engine on sale for £150. But Graham Stratfold, the museum's head of visitor services, warns that "you have to be careful not to ignore the low-cost, low-profit-margin, high-turnover end of the market. I suspect that most of our profits will still come from pencils and rubbers for schoolkids".

At the Lowry museum in Salford a spend of £2-£5 per child is not unusual, so he may be right. "But if you could sell the occasional beautiful high-quality live-steam models of a classic engine, for example, for £800, and contribute £300 to the museum, it would certainly help."

Museums can get away with such added-on value for a good reason: the cachet of a museum gives instant brand-recognition of the sort that marketing men give eye teeth for. At the RA shop the captive market includes not so many anoraks but more black-ties, and quite conceivably the odd millionaire. The average disposable income per visitor may therefore be higher. Even so, most of its patrons would be hard-pressed to come away with a whole Caravaggio, or even his hat. But if you do want to take something home for the dressing-up box, do not despair: they also sell, for children, the "Make Your Own Caravaggio Hat Kit", complete with slot-together shapes, ostrich plumes and plastic jewels.

Even expertly assembled I do not suppose it looks quite as good as the ready-made version. But it comes at a much more reasonable £7.95. You do not have to feel like a cheap-skate if you go for that option. There are always other museum shops you can visit with the £1,492.05 you have just saved.

The Genius of Rome 1592-1623 is at the Royal Academy of Arts, London, until 16 April.