There is not a specific number, but it is different for each operation, and can/will change over time. Cutoff grade is determined by calculating the production costs (mining, processing, etc.) and the amount you will earn from your product. Two operations near each other will likely have different cutoff grades.

It depends on the ratio of the Ta to Nb within the recovered coltan, it depends on what bi-products one is able to also recover from the operation (Sn, W, Sc, Zr, Au etc) & it depends on what price you are able to negotiate for each kg of your product.

Cut-offs around 150-200 pp is an industry standard to look out for, below this, you may be wasting your time. The above comments are all true. On alluvial/eluvial - overburden depth is critical. On hard rock, the shape of the geology is critical. Logistics, mining method, mineralisation particle size, radioactive content penalties, positive content value enhancement, fuel/power cost etc are all going to make a difference. You have to do some test work and create cash flows then sensitivity analysis for various factors. Many operations I have assessed are clearly not viable, but owners have spent money and time that would never have been wasted if simple initial analyses had been carried out.