Perhaps the most remarkable item at the new Baggy Green exhibition at the Bradman Museum in Bowral is the cap belonging to Adam Craig Gilchrist, Australian Test player No. 381.

It was presented to Gilchrist on his Test debut against Pakistan at Brisbane in November 1999. He wore it for the next 60 Tests before reluctantly replacing it with his second and final baggy green, worn until his retirement from Test cricket in January this year.

Gilchrist's original cap is so worn and battered it resembles an item of war memorabilia. It's an appropriate metaphor given the esteem in which the baggy green is held these days. Modern players consider them a sacred item, in some ways the sporting equivalent of the Victoria Cross medal. As with the VC, there are some who believe they should never be sold on the collectables market. Gilchrist, among many of the well-paid modern players, has said he would never sell his caps despite their obvious monetary value. Others appear to be less emotionally attached.

Ian Chappell, for example, has not kept one of the caps he received during his illustrious career. He didn't know where any of his were until he was told by his friend, Ashley Mallett, that one now sits in a glass case at the Glenelg Cricket Club in Adelaide where he began his first-class career.

It's only recently that these symbols of Australian cricket have been given their current status.

"Before the first sports auction held by Phillips in London in 1978, caps were generally not bought and sold but swapped or given away," Michael Fahey writes in his new book, The Baggy Green, co-authored with Mike Coward. Or even stolen, as happened to Greg Chappell when a spectator removed one from his head as he was returning to the dressing rooms at the Wanderers ground in Johannesburg.

Before 1978, old caps were likely to turn up in flea markets or antiques stores. Fahey reports that there have been 168 baggy greens offered for sale during the past 16 years, resulting in 122 sales at an average of $16,893 a cap.

Five Bradman caps have appeared and these deserve a separate category, such is their sentimental and monetary value. These average almost $160,000 each, including the top price of $425,000 paid for the cap worn by Bradman during the Invincibles Tour of 1948. It goes without saying that this is the highest price ever paid for a baggy green. The nearest challenger is the $180,000 paid for another Bradman cap from his 1947-48 season. Yet it seems remarkable that another Bradman cap sold for $7500 in 1995, an indication of how recently this market has taken off.

Top prices paid for lesser mortals include Keith Miller's 1954-55 cap, fetching $40,775 in 2006. Victor Trumper, with results of $28,750 (sold in 1997) and $83,000 (sold in 2004), is the closest in stature to Bradman, at least according to auction results. Trumper played in an era before the "baggy" style was introduced. The colour green was also different, a darker shade known as "gum-tree" green.

There is now considerable demand for baggy greens among sporting collectors around the world. The value depends partly on the status of the player who wore it and partly on when it was worn. The Invincibles Tour of 1948 is the most prized. Regardless of provenenace, most baggy greens now fetch more than $10,000.

This is a sharp rise from 1998 when Clarrie Grimmett's 1930 cap sold for a mere $7475 at auction. Given his stature you'd expect this to fetch at least triple that amount today.

The sharp rise in value is demonstrated by the progress of Colin McDonald's 1956 cap. This was first sold by Charles Leski in Melbourne in 1999 for $2990, then by Knight's Auctions in London in 2002 for $6571, then again by Knight's in 2005 for $7896 (all prices in Australian dollars). According to Michael Fahey's mathematics, that's a 264 per cent increase in six years.

Others have done even better. Wally Grout's cap achieved a 436 per cent increase in just four years.

As previously mentioned, there are those who find these figures almost offensive, including some of the modern players. Others insist that without private collectors and the collectables market, many of the older caps would not have survived.

Some of those in the Bradman Museum's exhibition were freely handed over for display by collectors, including such rarities as the one worn by Cyril Docker when he played for the 1919 Australian Imperial Force Team. It's navy blue with a black peak and the rising sun emblem but few would argue that it doesn't deserve a place in a baggy green exhibition.

There's been a surprising variety of emblem designs over the years, including the one-off white bicentennial cap worn by the Australian team during the lone Test against England at the SCG in 1988. Only 12 of these were produced and they rank among the most desirable of non-green caps.

Also of interest are the baggy greens worn by Australian women cricketers. Same cloth, same design (smaller head sizes, we assume) but differentiated from the male of the species by a reverse-colour riband. Belinda Clark's is on display at Bowral.

The Baggy Green exhibition continues at the Bradman Museum in Bowral until September. All images of caps courtesy of the Bradman Museum.

My collection

After graduating with an economics degree from the University of Sydney, Michael Fahey worked in the finance industry before deciding in 1993 that there was an opening in the sporting memorabilia market. He now runs Legends Genuine Memorabilia and, to coincide with the Bradman Museum's Baggy Green exhibition, he has written a book, The Baggy Green. It details the history of the Australian cricket cap as well as its second life as an increasingly lucrative collectable.

The first baggy green Fahey bought was one belonging to Mike Whitney, picked up in 1991 at his testimonial dinner.

Hundreds have since passed through his hands but the only one he has kept is a baggy white, signed by the Australian team. This was a special version produced for the 1988 Bicentennial Test in Sydney.

Fahey's book, which includes statistics on the many caps sold at auction since 1978, is available at bookstores as well as through the Cricket Publishing Company and Legends Genuine Memorabilia website (www.lgm.com.au).

$15,000; $100,000; $120,000

Ron Archer's baggy green, as worn on tour in 1954-55, last sold for $8000 in June last year. Archer may not be a household name but any caps from this period are increasing in value.

The great Victor Trumper is one of the few Australians to rival Bradman in the memorabilia stakes. This cap dates back to 1899, before the "baggy" period. It sold in August 1997 for $28,750.

Anything worn by Bradman is a blue-chip investment. The Don's 1946-47 cap sold for $88,000 in June 2003.

All estimates supplied by Michael Fahey of Legends Genuine Memorabilia.