My friend don't limit yourself only to what is physically possible. Go for a million Denarii, Solidi, or similar ancient coin. The solidus was something like 4 grams of gold for most of its existence so at current gold value 10^6 Solidi are over 400 million in gold alone, not to mention worth more than the gold in it as an artifact, though not for all of them. That would glut the market, but some as collectors items and turning the rest into bars, that's the way to maximize.
Don't just dream of the unlikely, reach for the impossible!