I installed one a few years ago. Bought a used one for $80, and it cost about $200 to rework the plumbing (had to use copper, since our building code is stuck in the 20th century). Like you, I wanted softened water for only some things. So the kitchen, master bath, and laundry get softened water, the kids' bathroom gets softened hot water but untreated cold water, and everything else gets hard water.
I wouldn't say there's been any financial savings, but rather it's a quality-of-life issue. Showers are more enjoyable, soap lathers and cleans better, the laundry is cleaner, the dishes are cleaner, there's less mineral film on stuff. The ongoing cost is minimal--a few bags of salt per year. I DIY'ed the project.