Reading the translator's notes, as someone who also lives in Japan, yeah I agree 1400 yen is weird to be considered "subtle luxury" as prices are rising everywhere due to inflation and we're talking Tokyo here. Also true that standard prices for a set meal dining out is around 1000-1500 yen, below 1000 is a good deal, around 500 yen plus minus up to 200 yen (300-700 yen) is low budget range of prices.
I can agree much more with the character's statement if the price of the set lunch mentioned is around 2000-2500 yen, for a French in Tokyo it's actually considered "affordable" but still above the average prices of dining out (for other types of cuisine) as I mentioned previously. Hence "subtle luxury".
EDIT: I actually just checked the actual restaurant's menu and reviews, most people ended up spent around 2000-3000 yen for lunch because they went for the full course including the desserts and others, so I agree with our main character's "subtle luxury" statement if we take the total amount on the bill.