Part of making a scroll requires you to provide the expensive material component of the spell in question. So for example, a cleric can make a scroll of Bless for 25 gp, but a scroll of Bless Water would be 50gp to include the 25 gp worth of silver dust.
But what about a scroll of Animate Dead? The base spell would be 3 x 5 x 25 (assuming 5th level caster) or 375gp. But the spell also requires an onyx gem, which depends on the hit dice of the undead in question. How do you know what that HD is going to be? For a human skeleton that's only an extra 25gp. But if I wanted to raise a ankylosaurus skeleton, it would be an extra 250gp. Does the scroll remember the value of the onyx gem I used in the making? If I use a 25gp gem, then use the scroll on an ankylosaurus, would it simply fail, or could I not even attempt the casting (as I wouldn't have the proper material components)? Can I supplement the extra components?
There's an even weirder scenario. If a 17th level wizard wants to create a scroll of a 9th level spell, it costs 3825gp. Now suppose that spell is Teleportation Circle. The extra amber dust bumps the cost up to 4825gp. But there's a flaw in this - the amber dust is essential to the spell, because it defines the circle! The amber dust is used to mark out a circle, which then becomes the teleportation circle. So if casting from a scroll, how do you mark out the circle? You need the dust at the time of casting, which the scroll doesn't allow.
"Activating a scroll requires reading the spell from the scroll. The character must be able to see and read the writing on the scroll. Activating a scroll spell requires no material components or focus. (The creator of the scroll provided these when scribing the scroll.) Note that some spells are effective only when cast on an item or items. In such a case, the scroll user must provide the item when activating the spell."
For the two examples, I would ask my players what they want.
Either the scrolls will have a set amount that I determine when I give them scrolls they find in adventures (or they choose when they make the scroll)
OR
The gems are not a part of the scroll, and they need to supply them when casting the scroll - and they will know this by reading the scroll ahead of time.
The teleportation spell says the circle is a set size: 5 ft radius. So the scroll could magically hold the dust, which would then cover the 5ft radius you designate when you cast the scroll.
what about a scroll of Animate Dead?
That's a question that has never been answered. Logically, the scroll would be "capped" by the amount of onyx used in its creation.
So if casting from a scroll, how do you mark out the circle?
That's a question that has never been answered. Logically, you create a makeshift circle some other way - with regular dust or a piece of chalk.