Good chapter. It feels a little rushed, but pacing necessitates that.
I liked the explanation of the conversion rate at the end. Even at 75:1 its still very undervalued assuming its gold, and it probably is at least 14K (58% Gold, 42% Silver) and more likely 18K (75% gold, 25% silver) though pure 24K is certainly possible, though it lacks the inherent durability of the 18K coins. Even so, assuming the coin is the size of a quarter and 3mm thick gets you a volume of around 0.61 cc volume/coin, which with a 75% gold content gets you .45 cc of gold per coin, that's still around $200 in gold. Melting it down for gold and selling it to muggles would be a winning proposition. Again, math failures by Rowling. There are so many errors in canon you may as well correct them as you go.
You are one of the few authors I'm still reading in HP fanfic as I transition back into writing original fiction once more. Hope to see more chapters for this and Distaff Side. Cheers.