There are things like frying pans that one can reproduce via a stock pattern. Armor (or probably anything wiht a high enough sale value to make this worthwhile for the wizard) isn't one of them - nobody buys plate armor "off the rack". It is a piece of custom work, like a modern bespoke suit, where you have to sit down with the client, take their measurements, discuss materials, and otherwise determine exactly what the customer wants. Every step of design a normal smith does *before* putting hammer to anvil still has to be done by the wizard.
It has to be done, but most of it does not have to be done
by the wizard. When you clear 750 gp on every sale, you can pay for a fairly swanky establishment and assistants/apprentices, and still come away with a hefty profit margin for a relatively small amount of work; maybe 2-4 hours per day, when you consider the demands of running a small business (which is what you are doing).
Alternatively, if you don't feel like being a business owner, you could attach yourself to a regular armorer's shop. Each day, the most expensive item in their queue gets assigned to you to
fabricate, and you collect a commission. The regular armorers make chain shirts and breastplates for the plebes, while you take the fancy clients.
No, you can't make 750 gp a day for 10 minutes of work, but it's a pretty good way for a 7th-level wizard with Smith's Tools proficiency to make a living. What it isn't is an economic game-changer.
And, like any professional, the price you pay is not just paying for the individual item, but all the education of the craftsman that goes into making the item. For a smith, you're paying for their proficiency with the tools, essentially. For the wizard, you are paying for that proficiency, *and* the years of hazard to gain levels to be able to cast the spell.
Also true, but in the context of a PC wizard, I think we are assuming that investment has already been made. At least, I've never seen anything in the chargen rules about student loans.
As a general business model, of course, training costs are very much a thing.