Tomatoes get big. They need a decent sized pot. I would recommend a 12" pot, defintely no smaller than 10".[0] Dirt aint cheap. "Cheap as dirt" wasn't formed from shopping at garden centers with bagged dirt, and you don't need a truck load for container gardening.[1] Tomoatoes need up right supports like cages, etc.[2] Seedlings are cheap individually, but each one needs a container.[3]
Well you're right that it doesn't scale down to single pots.
When I grew tomatoes in containers, I went for the 5-gal buckets from Lowes ($4), and bought soil in larger bags. Also your cage is more expensive than I recall paying.
On the plus side, most of those are one-time startup costs.
Our tomatoes are self-seeding at this point. We literally do nothing except pick them, and put away several dozen jars of sauce every year, from a couple neglected garden beds. It sounds like we have less trouble with pests here than you do though.
My original post said ~$50-$100 in initial setup, so you're not being fair to my original post. You need multiples of the things or similarly functioning things as posted. Yes, buying bigger bags of dirt is an option, or cheap plastic pots. If you're going to be doing this, you'll appreciate the clay pots as they don't bet brittle from UV exposure. I prefer clay as I just dislike plastic.
It's a typical reaction when suggestions that gardening can be more expensive than people realize. Yes, if you use the same pots, start taking seeds generationally, etc the price amortizes to near $0. That first year though can put a dent in a wallet. If you've been doing it for awhile, it's easy to forget.
We haven't even discussed getting into drip irrigation or similar. Each of the individual parts are cheap (tubing, sprayers/drippers, etc), but again it adds up with a timer etc.
To be abundantly clear, I'm not trying to dissuade anyone, but just giving realistic expectations. It's just like getting into Arduino and what not. The controller itself is cheap. The shields are cheap. Electronic components are cheap. Then you add up all your receipts and realize, you've spent more money than originally anticipated. It's just the nature of DIY.
Well to be fair to my unfairness to your original post, you'll also get a lot more than $5 of produce out of a single pot.
So, per pot, you'll spend $20 in the first year by my (updated) math, then $5-10 each successive year -- and you'll get about $15 of produce each season. Sometimes much more than $15. Cherry/grape tomatoes are expensive, and the plants are more fruitful than the larger varieties.
Drip irrigation is out of scope. A minute or two, per container, per day, with a hose (or watering can) while you check for hornworms is the operational expectation.
Anyway I don't disagree with your larger point. Little things definitely add up. I've been to IKEA!
I also dislike plastic, but I liked the form factor of the bucket (straight vertical sides) to pack multiples into a small space. I still have the buckets 15 years later, and I use them for all sorts of misc jobs around the garden.
There do exist these things called "dwarf tomatoes". Also, a large pot and a few good cherry tomato seeds can blow away everything in a supermarket with little flavor bombs.
I have determinate dwarf tomato plants (3 feet tall max), which I grew from seed ($2.99), planted in old milk crates (free) lined with plastic from old bags (free) and filled from one huge bag of potting mix (~$20-30), and this gives me half a dozen plants that are superior to anything in a supermarket. I staked them with tree branches trimmed from my trees (free).
I have grown them in the actual ground many years, eliminating the cost of the potting mix, but after a few years, you need to let the soil lie fallow to avoid blights, so this year it's the milk crate planters.
You can reuse most of those things for years, you can also use common household items to improvise replacements for cages and such. We bought little plants this year for 3 bucks each, we had no expenses otherwise.
[0] $10 https://www.homedepot.com/p/Pennington-12-in-Terra-Cotta-Cla...
[1] $5 https://www.homedepot.com/p/Miracle-Gro-Moisture-Control-8-q...
[2] $5 https://www.homedepot.com/p/Glamos-Wire-Products-54-in-Plant...
[3] $4 https://www.homedepot.com/p/Bonnie-Plants-19-3-oz-Early-Girl...
That's one plant, and we're at $25.