This translates to 5K Gold...a quite low gold content. With a gold/tin mix, I'm not sure this would even still look like gold. If the coin were smaller, thinner, or heavier...it could have a higher gold content.
There's a complication to this worth mentioning. According to 5E PHB p157 and 3.5E PHB p112: 1lb of gold is worth 50gp. And weighing a third of an ounce, 50 gold pieces equals one pound. Historically, including during the middle ages, money was worth the value of the metal it was made ofThis equality seems to exact to be a coincidence. A 50lb hunk of gold, or 50lbs..that a pound of gold coins wasis equal in worth the exact same thing. This was the international trade standard for muchto a pound of civilized historygold? While not any form of concrete evidence...money was worth the metal it was printed on. Before there wasit does raise a big international economy that determined relative currency values...if you changed the precious metal content of your coins, you changed the value of your coin relative to another nation's.secondary possibility:
Thus, itIt is distinctly possible that the coins are, in fact, 'pure gold'gold,' and thus their value is maintained by medieval standards. And the mismatch in weight and/or size is simply because the designers didn't bother with the math, and set the weight based on a gameplay decision rather than on realism. Simply saying "this seems like a good weight for a coin, in terms of how much weight an adventurer can carry...and we want our coins to be pretty large in size...so make them about the size of a half-dollar."
And if you're concerned about difficulty....D&D is set in a society that has reliable and sturdy steel production. Refining 'pure gold' is much easier, and was accomplished far earlier in history.
