An under-sink reverse osmosis system is sold as a box, but the invoice for a finished install depends on your cabinet, your sink, and your refrigerator more than on the box itself. Two quotes for the same model can differ because one company is planning around a crowded cabinet, a long run to the fridge, or a drain that needs extra work. Before you compare totals, know which parts of the job move the number.
The sink cabinet and the drain
Space under the sink decides how much work the install needs. A cabinet with room for the storage tank, the pre-filters, and the drain fittings usually goes faster than one packed with cleaning supplies and old plumbing. Ask whether the drain connection will use an existing drain or a new one, and whether a drain saddle, the fitting that taps into the drain line, is included or priced separately. Drain work also affects the air gap and the faucet setup, so get those choices written down.
The faucet and the hole
Most under-sink systems need their own faucet, which means either drilling a new hole in the sink or countertop or using an unused hole that already exists. Check who drills the hole, whether the quote covers the cutting, and what happens if the countertop is stone. Ask whether the faucet is part of the quote or listed as an option. A faucet with a different finish or reach can change the total more than the filter does.
The line to the refrigerator or ice maker
If the fridge or ice maker is on the plan, the distance from the sink to the appliance matters. Longer runs use more tubing, may need a fitting through a wall or floor, and take more time to route neatly. Ask whether the quote includes the tubing and the shutoff valve at the appliance, and whether the company connects the line or leaves that step to you or another trade.
Your existing supply and surprises
If there is already a filter or softener upstream, the installer may need to tie into it, change a fitting, or add a shutoff. An old supply line that is brittle or corroded can be replaced during the job, which adds cost. Ask the company what they expect to find before they start, and how they handle a surprise such as a fitting that will not seal or a shutoff that does not close fully. A quote that names those possibilities is easier to trust than a flat number with no conditions attached.
What lowers the bill without cutting corners
You can keep costs down by clearing the cabinet before the installer arrives, settling the faucet choice early, and deciding whether the refrigerator line is wanted at all. Ask for a line-item quote that separates the system, the faucet, the drain fitting, the tubing, and the labor. With a line-item quote you can see what changes if you drop an item, and comparing two companies line by line is far more useful than comparing two bottom numbers.