No, this calculation is incorrect. You forgot the most important principle of Quirrell points: unlike real estate, Bitcoin, and poorly drawn pictures, the value of Quirrell points (as denominated in every other currency) always goes up. This explains why you got such a low number ($400,000) for the value of 200 Quirrell points.
(heavy spoilers for ending of HPMOR):
HPMOR takes place in the 1990s, and importantly takes place before most people realized that the mysterious Quirrell was actually none other than the all-powerful nefarious amoral supergenius behind Lord Voldemort. Presumably, the exchange value of Quirrell points fluctuated over time -- low during periods when they only seemed useful for getting favors from an eccentric Defense Professor, high as the Defense Professor became increasingly well-known for his extreme competence and mysterious proximity to important events at Hogwarts, then reaching an astronomically high value when it seemed that Quirrell was on the cusp of achieving total domination over all of human civilization forever, then finally crashing to around the current value after Quirrell was defeated and imprisoned indefinitely until such time as he might be safely healed.
Economists debate whether the current market value of Quirrell points derives more from the possibility of receiving favors from Quirrell in future scenarios where he is revived (whether during a utopian far-future or a disastrous near-term return of Lord Voldemort), or merely from the fact that since only around 21 thousand Quirrell points were minted before its anonymous founder disappeared, Quirrell points form a sound monentary basis for a noninflationary store of value.
I have added spoiler-tags to your comment. Do not spoil the end of HPMOR on LW for unsuspecting readers!
In What Money Cannot Buy, johnswentworth points out:
The point: there is knowledge for which money cannot substitute. Even if Louis XV had offered a large monetary bounty for ways to immunize himself against the pox, he would have had no way to distinguish Benjamin Jesty from the endless crowd of snake-oil sellers and faith healers and humoral balancers. Indeed, top medical “experts” of the time would likely have warned him away from Jesty.
But Quirrel Points are issued by a wizard of significant knowledge and skill. A currency which comes with the advisement of that sort of expertise can bypass this sort of problem.
As well, if you look at the character's behavior, they're far more motivated by acquiring more Quirrel Points than they are by acquiring more galleons; they've recognized it as a more important source of power.
It is certainly possible that Quirrell Points are a better investment than lack-of-debt-to-Lord-Malfoy, but if so I would expect Harry to explicitly realize this rather than just entirely ignoring the option of exchanging them!
Now write the scene where Draco attempts to convince his father to accept Quirrel points in repayment of the debt.
"You see, Father, Professor Quirrel has promised to grant any school-related wish within his power to whoever has the most Quirrel points. If Harry gives his points to me, I will have the most points by far. Then I can get Quirrel to teach students that blood purism is correct, or that it would be rational to follow the Dark Lord if he returns, or to make me the undisputed leader of House Slytherin. That is worth far more than six thousand galleons!"
Lord Malfoy looked unconvinced. "If Quirrel is as smart as you say, why would he promise to grant such an open-ended wish? He warned you that Quirrel points were worth only one-tenth of House points, a popularity contest designed to distract fools from true politics and dominated by Quidditch seekers. For every plot you buy from Quirrel with your points, he will hatch a greater counter-plot to achieve what he himself truly wants. You must learn, my son, not to rely overmuch on those greater than yourself to serve as your willing agents; the power loaned by them is never free, and it is not truly yours in the end."
Epistemic status: Relies on the testimony of our trusted friend, Professor Quirrell
A thousand of your measly tokens pale in comparison to a single Quirrell point.
- Quirinus Quirrell
Harry and Hermione each had, like, 200 Quirrell points each, each of which was apparently worth at least 1000 US Dollars. $400,000 ≈ £300,000 ≈ 6,000 galleons. That’s a whole 10% of Harry’s debt to Lord Malfoy. I don’t really know what “pale in comparison” means, but it sure sounds like an order of magnitude. This important source of money is entirely ignored by every character in the story!