At a 10/12mtpa size, more drilling isn't needed to expand the...

At a 10/12mtpa size, more drilling isn't needed to expand the resource. There may be some infill for more definition of the chosen boxcut location, but this would come post-BFS. Further definition drilling would generally become an opex expense advancing detailed drilling around the pit shell as it expands once operational.
Other drilling will be required for geotech / soil testing where the plant & infrastructure is to be located, and probably along the haulage corridor to Broken Hill for road upgrades.
They "should" have enough detail to produce the BFS - so the costs will be largely consultant related. The trial plant data is already published so redoing that isn't necessary for BFS.
Pit optimisation will be a function of changing the settings in the pit optimiser software (I'm assuming Whittle or similar). The model will be already in place for the 20mtpa option - just rescale, hit a button and carry on. Yes- it's pretty much that simple. Trained monkeys can do some of this stuff.

If they can't get from where they are to bankable (not talking FID) for less than $3-4m (hopefully inside current cash on hand) then either the consultants are having a laugh or the management team doesn't have its eye on the ball.