RE:Impact valuation I like your conservative calculation/approach.
To me, the main point is...when was the last time a small explorer got 1b+ on some holes in the ground ? Long, long ago.
Imho...the intention of the Impact mm is clear...get the exploration costs back, get some small Wi (or royalty) with a carry to production and that's it...saves the overhead for decades and they can go on. Beside the mm wish, it's the most probable outcome for me too.