April 1991 price of silver was $3.9707/oz troy, gold was $358.38/oz troy. That's a 90:1 silver-gold ratio, ounce to ounce.
Now, the Sickle-Galleon ratio is 17:1. But a Galleon is larger than a Sickle, by a significant amount, as can be seen here; , and gold is denser than silver, by a significant amount. Assuming the coins are similar thickness, the Galleon is about 1.7 times larger than the Sickle, and about 3.1 times heavier. So the ratio by weight is around 5.4:1 silver-to-gold.
That means each cycle of arbitrage is a multiplication by, well, we'll round down to 16 for various transaction costs in our Fermi estimate. 100 Galleons becomes 1,600 after one cycle, and 1,600 becomes 25,600 after cycle 2.
Okay, that "couple times" didn't quite get us all the way there from 100. Harry needs to manage to get his hands on (considering the uncertainties on transaction costs) more than 234 but almost certainly less than 300 Galleons and run through the arbitrage cycle twice to get the 60,000 galleons.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 81, which should be published later today. The previous thread passed 400 comments as of the time of this writing, so it will pass 500 comments soon after the next chapter is posted, if not before. I suggest refraining from commenting here until chapter 81 is posted; comment in the 12th thread until you read chapter 81. After chapter 81 is posted, I suggest all discussion of previous guesses be kept here, with links to comments in the previous thread.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.) When posted, chapter 81 should appear here.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight,nine, ten, eleven, twelve.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: