"Each year, over 250 million carats of diamonds are mined worldwide. "
http://www.niza.nl/fataltransactions/docs/sierra_leone/1-1.htm

"The worldwide annual production amounts to approximately 100 million carats. Of the total it is estimated that 30 million carats are of gemstone quality."
http://anthonytan.users4.50megs.com/ah_factsheet.html

"It's estimated that by 2007 Canada will become the world's third
largest diamond producer accounting for a significant percentage of the world's diamond supply.
Its domestic diamond production in 2002 measured over 4.5 million carats increasing by 2003 to 11.2 million carats.
...
In 2001, over 3.3 million carats were mined in Canada's North Country at an average of $228 per carat- third highest in the world"
http://www.trigonexploration.com/canadian-diamonds.html

5 carats equal 1 gram. 5 million carats equals 1 tonne. 250 million carats equals 50 tonnes.
$228 per carat is $1140 per gram.

The diamond industry is interesting because it could be similar or vaguely related to a lunar sample return industry.
The diamond industry generally doesn't like to brag about how much quanitity it sells, so I'll take the high number of 250 miilion carats. Also the Canadian mines are probably going to produce well over there 2003 target of 11.2 carats by 2007 in which time they believe they will be third largest producer. So 250 million carats or more seems reasonable. So roughly in total dollar amount somewhere around 25-50 billion per year.

As I've said before I think 100 tonnes per year is a reasonable target for lunar sample return material- twice the amount of worldwide total of quantity of gem quality diamonds which are produced. The going price of lunar material is difficult to know, but I don't think it's been ever sold as low as $1140 per gram, I've heard $3000 per gram, as a price for material taken from the Moon. Though meteorites thought to be from the Moon can be significantly less- dollars per gram.
Anyways I believe lunar samples could sell at a wide range of prices and to various different markets- institutions- schools and universities, jewery, and souvenir and gimmick type markets.