Grape-size fragments from a famous 1947 iron meteorite shower in Russia called Sikhote-Alin are relatively common, but those originating from the moon are in greater demand.

The former sell for $10 while a lunar specimen of the same weight is worth $5000.

Meteorites from Mars are even more valuable on a cost-per-weight basis. A one-gram piece is worth about $1000 and their value is expected to rise, given the latest Mars rover landing.

In October 2007, a $1 million meteorite was offered for sale through Bonhams New York. This controversial item was a fragment of the Willamette meteorite, the largest found in the US and the ninth-largest in the world. Discovered in 1902, the Willamette was donated to the American Museum of Natural History. It weighed more than 16 tonnes, or did until a considerable chunk was removed in the 1990s and sold to a private collector in New York.

When put on the market in 2007, this off-cut was expected to fetch more than $US1 million before representatives of an American Indian tribe from Oregon claimed they were ''deeply saddened'' by the proposed sale of what they considered to be a sacred artefact. It didn't sell.

The best result from the Bonhams auction was a 99.5-kilogram Sikhote-Alin specimen, which sold for $US122,750 (including buyer's premium). Several others fetched more than $US50,000. But perhaps the real treasure was the Claxton mailbox, the only known private letterbox damaged by a space rock. It sold for $US70,000.

Interest in meteorites has boomed over the past 10 years and the Bonhams sale no doubt helped inflate prices - artificially so, according to some collectors. They say these well-publicised auctions are aimed at wealthy speculators and that more realistic values can be found at rock collectors' fairs. Two of the more famous are held annually at Tucson, Arizona and Denver, Colorado.

The US is the biggest market but there's a strong trade in Europe and serious collectors in Australia including Jeff Kuyken, who became fascinated by rocks from outer space as an 11-year-old.

Kuyken now works in fibre optics but his passion continues. He runs the Meteorites Australia website, which sometimes lists items for sale, and he is president of the International Meteorite Collectors Association (IMCA).

The latter's website is a useful starting point for anyone wanting to buy meteorites. You can check the bona fides of any seller who claims to be part of the IMCA there.

Kuyken says there have been many unbalanced reports on meteorite values in the media over the years.

''Many concentrate only on the highest-value pieces, which makes for an interesting story,'' he says. ''However, it has a follow-on effect, where people who may actually find one have completely unrealistic expectations of its value.''

The chances of finding a $1-million meteorite are, well, a million to one - or more. It's more likely that any you do find - in the real world or on the internet - will be worth closer to the $10 mark. And if you find them in Australia, you are limited by various state and federal laws.

In WA and SA, authorities have the right to seize meteorites, even if you find them in your own backyard.

The exportation of Australian meteorites is now forbidden under federal law. This has had a severe impact on values in Australia. Kuyken's estimate is a reduction by 50 per cent to 90 per cent. Specimens found in most other parts of the world can be imported or exported freely within Australia.

Reports of meteorite activity are relatively common. The most recent fall of note was at Sutter's Mill in California in April this year. The 800 grams of fragments found so far are fetching about $US2000 ($1920) a gram online. The largest single piece found weighs 44 grams.

There have been significant falls in Australia but the most recent, at Murchison in Victoria, was recorded in 1969. It is regarded as one of the most important events scientifically, but in monetary terms, Murchison meteorites are worth much less than more newsworthy fragments from California.