Your results are going to be varied. Store-bought clothes are consistent, for better or worse. Tailored (store-bought then altered), made-to-measure (make the measurements yourself), or handmade clothing is going to vary based on where you are, who does the work, and the materials involved.

If you find someone who does work you like (fit, price, quality) you'll enjoy it. If you don't, you'll waste time and money.

In general, at least here in the USA, I expect to pay about 0.1-0.5x the average store cost of an item to get it altered, 1.5-2x the cost of an item to get it made to measure, and 2-4x to get it fitted and made (with multiple fittings). Labor is expensive here. I don't make my own clothes because I don't have the time.

Fit varies. Tailors can make mistakes when they're measuring you. Sometimes two similar items can be altered the same way and fit differently. A good tailor helps a lot for alterations but that can only go so far, even with made-to-measure clothes. Full handmade/bespoke clothes with multiple fittings fit the best but are the most expensive.

One thing to note is some of how well an item fits is subjective or based on style. Most of my family can't tell if my clothes fit because their fashion sense stopped developing in the mid-90s. A lot of my friends can't tell if my clothes fit because they just don't care. You should figure out what parts of fit matter the most and prioritize those.

Quality/durability/materials varies a lot. A t-shirt can only get so durable, but a jacket can have many different features that make it last longer and look better. You can get away with lower quality fabric with better construction and stitching, but the fabric is the most visible component.

I try to get decent quality basic store-bought items altered (or just wear them as-is), and then get high quality handmade or made-to-measure items for clothes I don't wear as often.

I'd recommend starting with getting some of your existing clothing altered so you can figure out what you want from your clothing and your price range and then going from there.