I can't speak for every dev, but a common response I've seen to this question is cohesion. What I mean by that is an asset pack that has a particular style, where everything looks like it came from the asset pack, AND, this is likely even more important, quantity. The more assets either in the pack, or various volumes I can buy, the better.
Often times what prevents me from purchasing an asset pack is if there aren't simply enough assets because I know I'll hit a road-block and need to shop around some more and finding a style consistent with an asset pack from a different author is challenging. This is why Kenney's Assets are so popular, not only does he have a lot of different packs, but the packs he does offer, contain a lot of assets - and of course his are free.
I'd pay anywhere from $5-$150 depending on how much value I'm getting from the asset pack.