There were several different versions of what they found in the safe, in the box, & in the smaller box. Originally Thomas was going to find a wallet with ID and some 1000$ bills, but it was to convoluted to make it happen. A 1000 dollar bill is actually worth more than its face value now because they don’t print them anymore. The prices can go high enough that it’s an amount of money that could break the story. Large sums of money injected into the world of the working poor can shift a narrative badly in a lot of unintended directions. It had to be just enough to be a windfall, but not enough to cause people to wonder where they all got some cash. Gold is very stable as far as value is concerned, so it made for an easier thing to use, plus it makes sense for the man to have some stored in a box in his fallout shelter. These kind of gram bars have been widely available for long enough for him to have purchased a few. Coins might have worked too, but it’s easier to know the approximate value of a specific portion of something. Coins can fluctuate in value quite a bit, so it would require a lot more explanation in the comic to make it seem plausible.