As a branding consultant, you'd expect me to say that the brand is the answer. Often it is, but not necessarily in the example you gave.

You found a shirt you "loved", and it appealed to you (I suppose) BEFORE you had noticed anything about the brand label. If you found the expensive shirt alongside the less expensive shirts in the same store, then there was no difference in context. So we can't credit the branded environment either.

In your example, product quality itself is what mattered – material, design, etc. Branding might make a difference, but it didn't in this case.

Let's not forget the simplest things. Better products are more desirable and therefore can sell for more. The challenge is to place products in front of people who will not only want them but who are likely to pay the asking price. If you put the shirt back on the rack, then others might be dong likewise; and it isn't clear to me that their higher price point is working.

Branding can make a difference in how people regard a product. But product quality itself has an equal claim.