If you have very hard water then I think a water softener is definitely worth it. It’s not just your kettle that gets limescaled up it’s all the other pipes, taps, toilets, showerheads etc.
We have a posh hot water tap in the kitchen called a “Quooker” that provides instant hot water (because the house is a bungalow and the normal hot water tank is a long long way away) - when that was installed the bloke that sold it to us said it would be perfectly fine running direct off the (hard) mains cold water feed. It wasn’t - it furred up solid within 18 months and had to have its innards replaced (fortunately under warranty). It’s now connected to the soft water supply along with everything else!
The softener machine uses 4Kg salt blocks (two at a time) and I drop in a new block roughly every 2-3 weeks. It’s been in for a few years so I can’t remember how much it cost new.
The salt blocks cost £40 (€47) for 12 blocks (6 packs of 2), so the running cost is pretty low.