In a nutshell:
Clean, neat, modest, and affordable clothes that you can move freely in, without obscuring your alignment, work best for yoga classes.
You don't need a $52 designer yoga tank top, $88 yoga pants, and matching $102 hoody for your yoga classes. Sure, they're undoubtedly cute and they may get you compliments (on your clothes, not your yoga poses...). But maybe I'm just cheap, because I can think of a number of other things I'd rather spend my money on....such as taking more yoga classes and workshops.
I also teach yoga, and I prefer to have enough yoga clothing to get me through the week without having to wash them.
I'd spend a fortune in yoga clothes if I went the fashionable yoga wear route. It's not necessary!
Here I'm wearing a pair of inexpensive bike-style exercise shorts from the thrift store, and a semi-fitted t-shirt from one of my favorite yoga studios. Total cost less than $30.