What actually drives the price
The state of your source data
This is the single biggest variable, and it is invisible in the brief. A dashboard over a clean table with consistent keys is a day's work. The same dashboard over an export where the customer name is spelled four ways, dates arrive as text in two formats and 3% of rows are subtotals masquerading as data is three days' work — most of it before anything visual gets built.
If you want a cheaper quote, the highest-leverage thing you can do is send a genuine sample of the real data rather than a tidied version of it.
How specified the output is
"A sales dashboard" and "these six KPIs, filtered by region and month, in this layout" are different jobs. The second can be quoted precisely and built once. The first will be built twice, and someone pays for that.
Whether it has to keep working
A one-off analysis is cheap. Something that refreshes every month against a source you do not control needs validation, failure handling and documentation, and that is typically 30–50% on top of the build. It is also the part that gets cut from low quotes.
Who maintains it afterwards
Building something only its author can modify is faster. If you need your own team to maintain it, the naming, structure and documentation have to be deliberate, and that is real time.