There's nothing in the rulebook that directly covers this situation, but that's not to say we can't come up with a reasonable rule for it.
To start with, you're obviously providing the scroll to the NPC, in this case, worth 3,000 gp. Barring a very strange circumstance or mind control, they're not going to foot part of the scroll cost. Seems obvious, but it's worth explicitly stating.
Standard spellcasting cost, as you've written in your question, is Caster Level * Spell Level * 10. In your example, an 8th level caster would ask for 8 (caster level) * 8 (spell level) * 10 gp = 640 gp. They're doing no less work just because they're not providing the spell themselves, so the price won't be less. This would also cover enough attempts to expend the scroll, one way or the other.
But, we also need to address what happens if/when things go wrong. Since there's no hard and fast chart, the actual in-game effects are up to the GM. The NPC has undoubtedly heard of all manner of horrible things happening during botched scroll use. How many are fabrications meant to dissuade ambitious students is left as an exercise to the reader. Regardless, there's a decent chance (for our NPC, (15 + 1 - 8) x 5% = 40%) that the spell doesn't work and a 5-25% chance of a mishap. Best case (Wis 18+) that's a 2% chance of Bad Things (5% of 40% is 2%); worst case (Wis 10) it's 10%. That's a not-inconsiderable amount of danger for them to place themselves in, to say nothing of what might happen to you! I'd say that's worth hazard pay, so bump the estimated fee to 960 gp, a 50% increase.
Getting the NPC to agree at all will likely be a task in and of itself, especially at 4th level. They're going to want proof that you're capable of dealing with whatever effects might happen if things go wrong. At a minimum, that would involve healing any damage inflicted and removal of the spell should it affect the NPC. You'd also have to agree to hold them harmless should a mishap destroy the scroll or twist the spell's effect in undesirable ways. This doesn't include the danger to the NPC of unknown magical writings, such as sepia snake sigil or explosive runes.
This is based on the following clause under Spellcasting And Services:
Furthermore, if a spell has dangerous consequences, the spellcaster will certainly require proof that you can and will pay for dealing with any such consequences (that is, assuming that the spellcaster even agrees to cast such a spell, which isn't certain).
Your GM might let you negotiate down the hazard pay if you can show sufficient preparations or deep enough pockets (and a willingness to escrow reasonable healing/repair costs).
It's also wise of you to very carefully research the NPC before attempting negotiations as there isn't much you could do to keep a significantly more powerful spellcaster from just taking that 3000 gp scroll and leaving.