Short answer: Similarity-review cost depends on document length and whether you only need a report or also need editorial help interpreting overlap, citations, quotation, paraphrasing, and attribution issues.
A similarity percentage is not automatically a plagiarism finding. Context matters: references, quotations, standard methods language, and properly attributed material can all contribute to similarity.
Ask what tool or report is being provided and whether the service includes ethical revision support. A provider should not promise to “beat” a detector or disguise copied text.
The goal should be accurate attribution and original scholarly writing, not manipulating a score.