Three items that cost money but have saved far more.
Clothes line-this was cheap anyway. I really could do with a second! Nothing like clothes dried on the line. One trick I've learnt, recently, is to put on a wash last thing at night so it is ready to go out first thing in the morning. In England, in Spring and Autumn, this is often the difference between a load or two dry rather than needing to use the tumble dryer or have damp washing round the house.
My steel thermos. This was brought in a National Trust shop sale in Devon, on a cold February day. I take a hot flask of coffee whenever I'm driving a long distance or when having a picnic lunch. Brought coffee is so expensive-just work out how much even one brought coffee a week is over a year vs taking your own. I do occasionally buy coffee out but try to make this a very special treat not an every day or even every month event. I had conventional thermos flasks before but they don't keep the coffee so hot and with each flask there is some dreadful moment when the toddler drops it and there is a little tinkle. The steel thermos has been dropped and has a few dents to prove it but works just as well as ever. Good for my sanity!
The breadmaker-this was expensive. We calculated that a breadmaker would pay for itself in 100 breadmakings. That was before we found that we could make a successful loaf with value flour rather than strong flour. Of course, if I was better organised I could make bread in the oven in big batches and save more but I'm not. I spent several years trying and failing to make regular big batches. The breadmaker works better for us.
I would love to know how other people have spent to save.