I have a water softener. The total cost was about $3,500. It needs to recharge every 2,600 gallons used, and a recharge uses about 100 gallons. The recharge puts a lot of very salty water into the sewer, which I hear isn't something the water treatment plants appreciate (I don't know exactly why). It didn't change my mind, but it's something to be aware of.
For reference, my family of 4 uses about 160 gallons of water per day, excluding landscaping (I monitor usage with
this handy device, which also lets me shut off water remotely if needed. It supposedly detects leaks, but I don't have any experience with it detecting leaks, just false positives)
So essentially every 16 days, I have to throw away about 1/2 day of water. In other words, I use about 3.7% more water due to the softener (100 gallons thrown away per 2700 gallons pulled). It's probably a bit different than that, I believe the softener will recharge itself when it's within about 50 gallons of needing a recharge, but there are also times it gets to "0" and we pull more water before a recharge.
I put a bag of salt in per month on average. Those are about $8
As for drinking water, I used to have an RO system to pull the salt out of the softened water before drinking, but RO is very very pure water (less than 10 parts per million). That's fine, it's not going to hurt you, but it might not have much flavor or minerals. The RO system cost was around $500, but it was 2015. I imagine it's more like $1k these days. The tank and filters were under the sink. Some of the filters need to be changed every 6 months and the membrane was every 2 years. Different systems have a different number of filters. IIRC, mine had a pre filter, sediment filter, RO membrane, and post filter. The filters were usually about $30 each, the membrane was around $100.
Replacing them was not particularly difficult, but did take around an hour or 2 to replace. That meant no kitchen access and no drinking water. The first few gallons of RO water needed to be discarded, so that was another few hours of waiting, filling a pot, and dumping water. To make matters worse, there was one tricky hose connection that would leak if not installed properly. I was able to cure it with Teflon tape, but man did I hate trying to fix that. It happened more than once and it took me a few tries to fix it. Maybe I'm just bad at that stuff, but it sucked. You can pay a professional to do this for you. I think they'll charge at least a few hundred dollars to do this.
RO also wastes water. I didn't have an exact measurement for mine, but google says RO typically wastes about 5 gallons per 1 gallon of drinkable water. So about 83% is waste.
RO water will also come out at tap temperature. Obviously there are ways to cool it down, and obviously some people don't like cold water so it's a non issue.
The RO filters + tank will also take up quite a bit of space under the sink, so if you're a person who likes to use that space to store things, there's going to be a lot less space
After all of the headache with replacing the RO filters, cost of filters, possible cost of having someone else replace the filters, and reduced under sink storage, I no longer have an RO system.
I now have Sparkletts deliver drinking water. Their dispenser is $3.99 per month to rent and offers hot, room temperature, and cold water. It's $15 per delivery and $7 per 5 gallon jug. I end up around $70 per month. It's probably a bit more expensive than an RO system over something like 5 years. The only hassle with this is every month or so I have to carry 8-10 5 gallon jugs from my front door to the dispenser, but I mind this much less than the RO hassle.
Also, as a side note, a good landscaper will know how to run water from the street connection to the irrigation lines, which will be untreated water. I hear plants don't like softened water.
