What exactly does the price include?
Ask for the inclusions in writing: draining, removal, disinfection, rinsing and a final check. Two quotes may look close while one drops disinfection or replaces removal with a quick rinse. What is not written is hard to claim later.
Also see whether the price covers multiple openings or extra tanks, and whether any fees apply for carrying tools up floors.
A clear, detailed offer beats a vague cheap one because vagueness turns into disagreement at execution.
