What affects the quote?
A narrowly defined records search and several days of surveillance are different assignments. Costs depend on the question, the time and locations involved, travel, records fees, and the kind of report you need. Rush work or court appearances can change the scope. Ask for a case-specific estimate rather than treating an advertised hourly rate as the total.
Compare the same scope
Ask whether the quote is flat-rate or hourly, how a retainer is applied, and whether mileage, records, reporting and testimony are included. Compare proposals against the same deliverable. A low quote for a basic lookup is not comparable to a quote for verified findings and a written report.
Start with a spending limit
Describe the smallest question that would help you make a decision. Ask for a first milestone and a written approval threshold before the assignment expands. If the initial work answers the question, there may be no reason to spend the rest of the budget.
General information, not legal advice. For advice on a specific matter, speak with a qualified attorney.