"I like your outfit. Affordable?" How is this an insult?

A big indicator of success is arguably wealth, and a big indicator of wealth is conspicuous consumption, i.e public displays of luxury items/expensive goods.

A lot of very shallow, very vain wealthy people believe that it is appropriate (and important) to publicly and conspicuously display items that immediately indicate their financial/social status; such as jewelry, finery and expensive clothing. A lot of Rap Music has unfortunately fallen into this mindset, and Wiz Khalifa can probably put it a lot better than I:

The bigger the bill, the bigger you ball The bigger the watch, the bigger the car, the bigger the star The bigger the chain, the farther you go, you already know The bigger the bank That's more hoes, nigga And I done spent a quarter milli on clothes

It's a very old fashioned (and for the majority of humanity) embarrassingly outdated worldview, but one which Envy Adams undoubtly subscribes to.

By saying someone's outfit is affordable, she is intending to insult them by pointing out that their clothing could be easily obtained by someone without wealth, ergo without success, and is trying to undermine her personal style by highlighting it's lack of exclusivity.

I think it is suggesting that her clothes are cheap, which is supposed to be offensive.