At the moment, sustainable fashion is more expensive than fast fashion. However, a great way to help combat this is actually to keep buying sustainably. As the demand for sustainable fashion increases, the production of eco-friendly fabrics will also increase, therefore the price will start to lower overall.
Although the popularity of sustainable fashion is growing, it is still a niche industry.
For big companies to change their way to more environmentally-friendly is expensive, at least in up-front costs, which doesn't allow makes for businesses necessary changes.
Unfortunately, to date, it's cheaper for manufacturers to use new plastic for fabrics and ignore the usage of eco-friendly natural materials, that is one of the reasons why sustainable brands become more expensive.
Also, choosing factories with ethical manufacturing drives up costs significantly for sustainable fashion brands. Ethical manufacturers pay their employees well and offer important human rights such as the right to unionize, have limits on working hours, and a safe working environment.
So, sustainable brands never will be as cheap as fast fashion brands (i.e., H&M or Zara), but together we can close the gap between fast fashion and the sustainable fashion industry.