Length and depth of the trench, soil type (digging rock or heavy clay is slow and dear), access for machinery versus hand-digging, the discharge solution (a soak well or stormwater connection adds cost), and whether a licensed plumber and council approval are involved. A short, shallow run on a sandy block with good access sits at the cheap end; a deep interceptor in rock that needs an engineered discharge sits at the top.
A handy homeowner can install a simple lawn-drainage run for the cost of materials, gravel, ag pipe, fabric and hire of a small trencher. But the moment the drain protects a building, crosses a boundary, or connects to council stormwater, you're into licensed-plumber territory in most Australian states, and getting the fall or discharge wrong can make the problem worse.
Ask each installer to spell out trench length and depth, the pipe and gravel spec, whether geotextile fabric is included, and exactly where the water discharges. A cheap quote that skips the fabric or hand-waves the discharge point is usually cheap for a reason. Get the discharge plan in writing.