The practical answer to where you stop: buy samples, in pairs, and drink them side by side.
Most decent sellers offer small sizes. Buy the same type at two price points and brew them together, same water, same leaf weight, same time, two cups in front of you. That comparison tells you more in twenty minutes than a year of drinking them separately, because tea is very hard to judge from memory — the gap between two cups is obvious and the gap between this week and last month is not.
What you will probably find, and what most people find:
- The step from the cheapest to the middle is large and obvious
- The step from the middle to the expensive is smaller and more specific — usually more complexity across infusions rather than a better first cup
- Above that, differences are real and require attention to notice, and they are the first thing to disappear if you are drinking while working
That middle plateau is where most regular drinkers settle, and there is nothing wrong with settling there deliberately.
One thing worth knowing: you cannot judge an expensive tea with the brewing you use for a cheap one. A tea whose value is in its fourth and fifth infusion looks like an expensive disappointment if you brew it once in a mug. If the two you tried were brewed western style, that alone could explain your result.