"It wouldn't be cost effective to try and bring it back to Earth. The last I paid any attention, gold was selling for somewhere around $400 an ounce. It costs more than that to launch payloads to the moon or Mars, and it isn't cheap to launch anything from those places back to Earth. "

If rocket fuel on the Moon could be bought for $125 per ounce ($2000 per lb), then it's somewhere close to being possible- assuming it's easy to mine the gold. Of course at the current cost of at least $10,000 per lb, it's a given that's not going to work. Now, the going rate of any lunar material (dirt) is around $3000 per GRAM. The price of anything is the price that people will pay for something. What people will pay for lunar material if large volumes are available (+100 tons per year, worldwide) is unknowable. One can make wild guesses, just like one can make wild guesses about what the Dow Jones will be 10 years from now- or the price of gold will be in the next decade. It's a pretty good bet that gold's value will be at least $100 per oz in 10 years time. I think it's a pretty good bet that the first 100 tonnes of lunar dirt which is returned to the Earth will sell for at least $400 per oz (or about $15 per gram). Of course you could add value by being somewhat selective in what dirt you ship back to Earth- 10 samples from 10 location would probably have more return value then all of it coming from one area.
Now, this, of course, would not pay for all lunar operations, but shipping flowers or fruit via air does not pay for the entire airline business.