NST and Acra Venture Quarterly, page-19

My calculation is about three times yours. That calc was endorsed by John Downes, the leading world expert on caesium formate, on this site. John Downes posted here as "themanfromwelo". I think you are referring to capital cost of extraction. This would be very low, probably not more than $2M. The mining would be by contract and direct ship. Crush and possibly screen, ship out in containers. The cash cost should be below $200 a tonne, so costs are nearly irrelevant.