At three in the morning the question is not which device is cheapest. It is whether the thing on her night route is running, and whether you know who pays for it when it stops. Start there, then work out the cost.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
What to check first
Find the paperwork or the account page and read four lines in this order: whether you are renting or buying, the monthly line that repeats, the notice you must give to end it, and the fee that starts if the unit comes back late or damaged. If any of those four lines is missing from what you were given, ask for it in writing before you accept the device.
What pushes the total up
Monthly service tiers often bundle features she will not use on the bathroom trip, such as extra sensors in rooms she never enters at night, a camera, or alerts sent to more than one phone. Each added sensor or alert can be a separate recurring charge. A long contract term turns a small monthly line into a commitment that outlasts the reason you started it. Replacement pads, batteries, and cords are often sold separately, and a unit that needs an installation visit adds a line of its own.
Late return and damage charges are the quiet ones. A rental that comes back with a missing cord or a cracked housing can cost more than a month of service. A second installation visit to fix a placement that did not work the first time is usually billed separately too.
What pulls the total down
Pay for one unit that covers the single route you are worried about, not the whole house. Ask for the shortest term available, and ask in plain words whether the service can be paused, moved, or ended with notice. If you expect she will need the device for a long stretch, compare the purchase price with the total of the monthly line over that same stretch. Buying outright often wins on paper, but it brings its own warranty dates to read first. Renting often makes more sense when you do not yet know whether the night route will stay the same.
Compare the total, not the monthly line
Add up what you expect to pay across the months you think she will use the device, plus setup, plus any return or replacement cost. Then add the time you will spend on the phone changing settings. Compare that number between two offers, using the same months for both. A device that looks cheap per month can be the expensive one by the end, and a device that looks costly can come out ahead if it carries no monthly line at all.
What to write down
Write the name on the account and the phone number the company uses. Write the contract length, the notice you must give to end it, and the date that notice is due, counted from the day the device arrives, in your own calendar. List each recurring charge with its amount and the day it posts. Record the serial number, the return deadline, and the name of the person who answered each time you called. Keep the printed terms in the same folder as the medical paperwork so the next person who opens it can find the end date without searching.
Whether the device fits her walk, her balance, and her bathroom trip is a question for her doctor or her therapist, not for a price list. Bring your written list to that appointment so the cost and the clinical question get settled in the same conversation. If you want local places to ask about equipment and loan closets, the local help page lists where to start in St. Louis.