First, sustainable fabrics are made out of low-impact organic crops. These crops are grown typically without pesticides, or synthetic fertilizer and are not even modified genetically. There are strict guidelines for such organic fabrics and that’s why they are more expensive than any other synthetic material or recycled fabrics.
Second, how people are treated and paid who are designing or making sustainable clothing, also makes a huge impact on the price tag. The brands that are honestly working on sustainable ethnic wear have sweatshop-free and child-labor-free premises. They consider everything; health, environment, and social impact.
Brands of fast fashion keep throwing out new styles trend after trend. They are cheaper so that you fall for them and of inferior quality so that you discard them within short periods. Sustainable clothes last longer and since they are not based on-trend, they can be worn n number of times.
Now tell me, when you are buying fewer clothes (slow fashion) and the ones which will last longer because of higher quality, won't they justify your action of paying a little higher than the usual course of shopping? Also, we feel that the amount spent on the planet, people, and overall prosperity is not an expenditure, but rather an investment.