For the strictest possible reading, only the "text" of the spell can be changed, and given the "power word kill" takes up nine pages, obviously text isn't the only thing on the page.
What else could there be, besides text? Spellbooks aren't magical items.
Sure, when you actually
cast a 'power word' spell, you're only saying one word, but what sort of twisted geometries must one run through their mind to achieve such power? What manipulations of magic must be expressed on the page? In what manner? None of that is ever gone into in any sort of detail (not in any of the books I've seen), though it is hinted at to be quite the laborious process. Should we look to Jack Vance's writings as some indication? That's the closest thing I know of. Those pages and pages are instructions for how to cast the spell. Maybe it is just that one word. It could require tonal variances with such an exacting degree of accuracy that it takes pages to explain the vocal chord movements required. That's another thing: if it requires a verbal, somatic, and material component, the instructions have to include what to do with each of those (which hand movements, vocalizations, and where to shove that guano). If anything, it should be surprising that so much fits on one page.
A first level wizard with eleven intelligence (about what we see in the real world when it comes to your normal worker carrying out a task) takes an hour to prepare three cantrips and a first level spell. That's twelve minutes each per cantrip, and twenty-four minutes for a first level spell. I don't know what, exactly, he's doing that's taking so long, but it must be a lot for him to get that one Sleep into his head.
Making some basic assumptions, I'll say he reads one hundred words per minute and all he needs to do to prepare a spell is read it. We also know that a page must contain an entire first level spell. That gives us 2,400 words he has to read (being generous; it could be three or four times as much, considering his training). Maybe there are diagrams or somesuch, but who's to say? What font size does that have to be?
'Normal' black ink costs 8gp an ounce. Other colors cost twice that. How many ounces of ink will he need for one page? It has to be a large page. But he also needs super-heavy-duty ink that won't blur when he writes his teeny-tiny words. In a pseudo-medieval world, it's not surprising that it would cost so much for the ink. Especially when you take into consideration the demand for this stuff. Most wizards will need it (unless they want to carve spells on bones or get it tattooed on; and the amount of information a finger bone can hold when compared to a whole page of a spellbook is nonsensical).