Short answer: Scientific editing generally costs more than basic proofreading because it may involve technical terminology, clarity of scientific argument, consistency across sections, and field-aware editorial comments in addition to language correction.
The quote should reflect the manuscript’s word count, research discipline, current language quality, complexity of tables or equations, target-journal requirements, and deadline.
Confirm whether the editor will address only grammar or also clarity, flow, terminology, abstract consistency, figure/table references, and journal-facing presentation. These are materially different scopes.
No scientific editing service can guarantee peer-review results or acceptance, so avoid providers that include an acceptance promise in the price.