Mark Dykeman indicates that at regular work, he fills up a 180 page notebook and at the relatively steep cost of notebooks, he's paying $1-2 a day for paper.
This naturally brings up the idea of what it might cost per day in index cards for some zettlers' practices. I've already got some notes on price of storage...
As a rough calculation, despite most of my note taking being done digitally, I'm going through a pack of 500 Oxford cards at $12.87 every 5 months at my current pace. This is $0.02574 per card and 5 months is roughly 150 days. My current card cost per day is: $0.02574/card * 500 cards / (150 days) = $12.78/150 days = $0.0858 per day which is far better than $2/day.
Though if I had an all-physical card habit, I would be using quite a bit more.
On July 3, 2022 I was at 10,099 annotations and today May 11, 2023 I'm at 15,259 annotations. At one annotation per card that's 5,160 cards in the span of 312 days giving me a cost of $0.02574/card * 5,160 cards / 312 days = $0.421 per day or an average of $153.75 per year averaging 6,036 cards per year.
(Note that this doesn't also include the average of three physical cards a day I'm using in addition, so the total would be slightly higher.)
Index cards are thus, quite a bit cheaper a habit than fine stationery notebooks.