Work of the classic cartoonists, such as Giles and H M Bateman (who is said to have inspired Ralph Steadman), has doubled in price over the past five years. Scarfe, Steadman and Heath Robinson (whose name has passed into the language) are all in demand, attracting high prices, and many collectors want to buy original works by Gary Larson, famous for The Far Side, which he does not generally release for sale.
Original cartoon artwork of this kind is usually in pen and ink, though some is in charcoal, coloured ink or watercolour. The best cartoons are recognised by their admirers as high-quality art.
Chris Beetles's gallery in London has a 700-item Christmas exhibition until 19 January, with items ranging from Larry cartoons at pounds 100 to the work of Ronald Searle, selling for more than pounds 2,000.
At John Rae-Smith's Islington gallery he is selling a Giles original for pounds 1,600. He recently sold a Heath Robinson illustration for pounds 3,000. But there are other pieces selling for pounds 20. Mr Rae-Smith says the average price is pounds 125.
"Cartoon and comic artwork from the early part of this century is very valuable, but there is very little contemporary artwork that will have much value for a while," says Paul Gravett, the administrator of the Cartoon Art Trust. "Giles's cartoons are going up and up. When he died, everything he did went up a couple of hundred. He got virtually everything back from the Express. Very few have gone on the market, which has put up their value. They sell for pounds 500 to pounds 1,000 for a regular black and white Giles. Some will sell for pounds 3,000 to pounds 4,000.
"A lot of good cartoonists, who are not household names but whose work is very good, are worth investing in for the future," Mr Gravett said. He predicts that the work of 1930s and 1940s cartoonists Edward Hynes (who produced the early, non-pornographic covers for Men Only), Leslie Grimes ("a wonderful charcoal technique") and Robert Sherriffs ("overlooked and influential") will all rise in value.
Cartoons that mark important events, such as the fall of Margaret Thatcher, are also likely to appreciate, Mr Gravett believes. He warns, though, that as events fade into obscurity so the meaning, and the value, of political cartoons inspired by them can fade as well. Often cartoons that are more general social comment retain their meaning and their price better.
Note too that many widely available 19th-century cartoon prints, although often relatively cheap, are not originals but limited print runs and are unlikely to have the same scope for investment gain.
The Cartoon Art Trust gives advice about buying cartoons, and hosts evening classes and lectures, as well as regular exhibitions. It holds an annual auction each autumn that is hosted by Bonhams.
September's sale included a Doonesbury strip by Trudeau (whose cartoons are syndicated to the Guardian), that sold for pounds 275, a Steve Bell cartoon (also in the Guardian) for pounds 140, a portrait of the Cabinet by Riddell (whose cartoons used to appear in the Independent on Sunday), that went for pounds 325, and a Peanuts strip from Charles Schulz that sold for pounds 1,200. Peanuts cartoons sell well because they have a large following and few come on the market.
Cartoons often appear in sales at the big auction houses. Cartoon originals of Winston Churchill, dating from the 1920s and 1930s, sold at Bonhams last week for between pounds 100 and pounds 220, considerably more than their guide price.
Politicians and celebrities commonly buy cartoon originals direct from the cartoonists - although some cartoonists refuse to sell to politicians they despise. The public could try the direct approach too. While a few sell their work through dealers, most will respond to written offers sent to their newspapers, magazines or book publishers.
Steve Way, one of the cartoonists who produces the Generation Why strip in the Independent, says he would welcome more offers. Mr Way says he has sold around 20 in a year. "It's nice to sell them - otherwise you end up with a shirt-box full and you don't get any contact to know whether people like them or not."
Jacky Fleming, whose feminist cartoons are published by Penguin, has sold her cartoon originals for between pounds 50 and pounds 100 each, although she would need a better offer to part with her favourites.
For those who are more interested in viewing than in buying, there is an exhibition of Giles's cartoons at the Glynn Vivian Art Gallery in Swansea until 14 January. Some are on loan from the Duke of Edinburgh, who is a keen collector.
q Cartoon Art Trust, 0171-405 4717; Glynn Vivian Art Gallery, 01792-655006; Chris Beetles Gallery, 0171-839 7551; Rae-Smith Cartoons, 0181-348 6569.