"Not to throw a practical damper on things, but supply/demand has a lot to do with price. Once you started returning tons of lunar material, its novelty and value would diminish exponentially. So, I don't think you could count on maintaining a high price for regolith. "

As I said 100 tons of lunar sample per year is about twice as much as is sold in gem quality diamonds [industrial diamond is different issue].
Of the 100 tons of lunar material per year a portion of this might be gem quality, so let's say in first year there is a ton of gem quantity lunar material [1/50th of volume of diamonds being sold worldwide] and this increase over time: 2, 4, 8, 12, 18 tons and then stays around 25 tons per year for a decade or so.