That's right -- when the 3He concentration in the regolith is only 10 ppb, you can't spend much energy per kilogram of regolith or you go energy negative. And the payback has to high, since energy on the moon is going to be rather more expensive than energy on Earth (due to the cost of building and operating things on the moon.)
Once the 3He has been extracted and concentrated by eight orders of magnitude, the mass-dependent energy costs become much more manageable.
One thing to consider about lunar 3He is that all our measurements have been at fairly low lunar latitude. It's possible there's higher concentrations in the regolith near the poles, since the rate of diffusion of implanted 3He should be strongly temperature dependent.
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Once the 3He has been extracted and concentrated by eight orders of magnitude, the mass-dependent energy costs become much more manageable.
One thing to consider about lunar 3He is that all our measurements have been at fairly low lunar latitude. It's possible there's higher concentrations in the regolith near the poles, since the rate of diffusion of implanted 3He should be strongly temperature dependent.