IIRC doing this in the late 80’s/early 90’s it seemed to work out best at $10/C bill which would be $20/C bill today if the C bill held its value while the dollar did not. Of course in the Battletech timeline the C bill doesn’t have a constant value for a variety of reasons so maybe just stick to $10/C bill.
Except, as stated, since the story mentions a very specific number, given yesterday's gold price, it is very specifically the case that a C-Bill is less than, or approximately, $2 USD today.
That's a danger in making a real world comparison in the story.
It’s noted explicitly in a few places in Battletech lore that gold there is less valuable than it was pre spaceflight. It’s relatively easier to get gold in space than it is when you are stuck on one planet, and it no longer has an appreciable value as a currency reserve. That leaves its value is jewelry, which is also reduced since its relatively more available. It’s not valueless by any means, but nowhere near as ‘inflated’ as our values for it are.
The story is Eyestorm in AGoAC by William H. Keith Jr. It builds on the story from the Beginner Box. I love his stuff generally, but here, he makes 2 mistakes. The first is by stating 12.4 kg is approximately half a million C-Bills, and the second is explicitly calling out at the end of the story that the gold can be used for electronics. The initial plan in the story is to use the mined gold as a reserve to get around using C-Bills and Comstar.
Overall, I wish he'd just made it Unobtanium, or Hypergold, or something. It's too specifically tied (not only to the real world price, but) to real world applications (currency reserve, electronics, etc. - pretty much the opposite of what you said).