Here is a thought that took me an unreasonable amount of time to have: a thing you already own is still costing you money.
The obvious costs
Some are easy to see. The car needs insurance. The flat needs to be big enough to hold the furniture. The bicycle needs a lock, and then the lock needs replacing, and then there is the small annual sum for the thing that keeps the chain from seizing.
When I moved countries I paid, by weight, to bring objects across an ocean. That is the clearest version of this: I wrote a cheque whose amount was determined entirely by how much stuff I had accumulated in the preceding decade.
The one nobody counts
Attention. Every object you own is a small open loop. It can break, be lost, need cleaning, need a decision about whether to keep it. Individually these are nothing. Two hundred of them is a background load you stop noticing, in the way you stop noticing a fan until it switches off.
The mandoline I mentioned in the kitchen post cost me its purchase price once and then a small tax every time I opened that drawer for four years and saw it there, unused, faintly accusing.
What I do with this
Mostly it has changed how I buy rather than how much I own. Before buying something I try to name the ongoing cost — storage, maintenance, the decision cost — out loud, in a sentence. If I cannot name it, I do not understand the purchase well enough to make it.
It is not a rule and it does not always work. I bought a second dripper last year for no defensible reason at all.