Hiring a carpenter is very expensive. The cost of tools is tiny compared to the cost of their labor.

(I'm assuming that we're talking about saving money in a way that counts your own time doing something you enjoy as free.)

Exactly that. Top brand tools are expensive, but lower mid range these days is surprisingly cheap, and even for one off projects might "pay for itself". And they get cheaper. Labor costs are increasing. Obviously this comes with few caveats: a) you have to enjoy what you do, so the labor is free and you treat the time as me-time and b) you actually know what you're doing... running in to a disaster and then getting someone to fix it can be very-very expensive :)

A carpenter charges like $50/hr. A tablesaw alone would likely set you back more than the labor for some simple shelves. Then they can get better deals on material, they do higher quality work, and they do all the parts that aren't fun, like cleaning up at the end.

Also your time still has value even if you enjoy what you're doing. You could be doing other hobbies you enjoy more, or other aspects of the same hobby, or making money so that you could afford other opportunities instead. You may enjoy woodworking enough that you're willing to forego the value of that time to pursue it, but that doesn't make it any more economical.

With your estimates on payback time, I think you might be underestimating how many things I've built around the house ;)

Some examples:

* At least five sets of shelves, the biggest of which was https://www.jefftk.com/p/built-in-shelves

* Piano stand, so I could put my keyboard at just the right height on top of a wagon

* Folding monitor/TV, that is pretty sturdy and well protected when it's away: https://www.jefftk.com/p/folding-couch-monitor

I expect to keep doing this, and most of my tools aren't even halfway through their useful life.

There are some downsides, and you're right that a professional would make a lot of these things look better, but there are also serious downsides to working with a professional aside from the cost. It can be quite hard to get them to come when you want, they may not build exactly what you had in mind since communication is hard, and some people will cheat you (https://www.jefftk.com/p/brendin-lange-is-a-scammer)

Over your lifetime, you might need 5 or 6 sets of shelves. Buy yourself a porta-tablesaw for about $400-$500 (or cheaper on craiglist etc), learn to use it, and chances are that somewhere in the middle of your life, the cost equation will have flipped in your favor.

You don't need a tablesaw to make some shelves. You'd get very far with a hand held jigsaw. Can be stored in a drawer and costs around 100€ for a decent brand. You just need to be a lot more careful to get things perfectly straight, compared to a circular saw.

That values storage costs at zero. Certainly not true if you live in a duplex, let alone an apartment!

Absolutely fair point. That's definitely a very different situation. Probably one of the reasons I doubt I could ever live in such a situation, at least until I'm 75 or so :)