When you compare water softener bids, the unit itself is only part of the number. The rest is the labor to connect it to your home, and that labor depends on conditions you can check before anyone quotes you. If you compare totals without comparing the assumptions behind them, you may be comparing two different jobs.
Where the water comes in and where the drain goes
Start with distance. The farther the unit has to sit from the main water line and the drain, the more pipe, fittings, and time the job takes. Walk the path from where the main line enters the house to the spot you expect the softener to go, and note how many turns and walls are in the way. Then do the same for the drain. A standpipe or drain connection close to the install spot is a short job. A drain that has to be reached through a finished floor, or that needs a new line run to it, is a different job.
Ask each installer how many cuts and new connections their bid assumes. Each joint is work, and a bid that assumes four connections is not the same as one that assumes eight. If the numbers are not written down, ask for them.
What the pipes sit behind
Access changes the labor more than most homeowners expect. Pipe that runs across an open basement joist is easy to reach. Pipe inside a finished wall, under a slab, or in a tight crawlspace may need an opening made, patched, and finished. Before you accept a bid, ask the installer what they expect to open up and who repairs it. If the answer is that they will see when they get there, ask what happens to the price if the pipe is somewhere they did not plan for, and whether they will call you before cutting into anything.
Power, space, and the old unit
Some installs need a nearby outlet. If there is none where the unit will sit, find out whether the bid includes running power or whether you will need an electrician. Next, measure the route the unit has to travel, including doorways, stairs, and turns, because a unit that has to be carried down stairs or through a narrow door takes more labor than one that rolls straight in. Also ask who removes the old unit, whether it is included in the bid, and whether disposal is part of the quote.
Existing equipment and add-ons
If your home already has a filter, a pre-filter, or other equipment on the line, ask whether the installer will move it, reconnect it, or work around it, and whether that is in the bid. A bypass valve is another line to check. Find out whether it is included, whether it is installed or only supplied, and who is responsible for it later.
How to compare the bids
Give every installer the same list of site conditions and ask each one to answer in writing: the distance from supply and drain, the number of connections, the pipe location and access, the outlet situation, the removal of the old unit, and the bypass valve. Then compare the answers line by line. If one bid is much lower, the first question is usually which of these items it leaves out or assumes away.
For the broader picture of what makes up a total price, the Missouri Water Softener Buyer's Guide walks through the stages of buying and installing a unit, and it can help you decide which questions belong on your list before you request quotes.