The biggest cost driver isn't screen count, it's integration complexity. An app that talks to your existing CRM, accounting system, or inventory platform costs meaningfully more than one that stands alone, because every integration point is its own piece of work that can break and needs testing, not just a line on a feature list.
The second biggest driver is how much of the logic is genuinely custom versus how much is a standard pattern, login, list views, basic forms, that exists in some form in nearly every app. A project that's mostly custom business logic specific to how your operation actually runs costs more than one that's mostly standard screens with your branding on it, and that's often invisible until someone actually scopes the work.
We quote against your actual project, not a category average, which means the number you get reflects what you're really asking for, not a guess padded to cover the vendor's risk on an unclear scope.