So the simple answer is - if it's really cheap, it's unlikely to be sustainable.
Cheap clothes use cheaper materials, cheaper labour and cheap processes. They are built to display and to sell, not to last. Making clothes is a complicated process, so if you're looking at a cheap garment, the chances are - it won't last. The goal of this suggested action is to limit the quantity of new clothes that people buy. You can do this in the following ways:
1. Buy less and be happy in the clothes you own
2. Buy from second-hand shops, re-using clothes that are already in circulation
3. Buy higher quality clothes, that will last for a long time
Any of these actions helps address the over-consumption we see in the fashion industry, where the global emissions are larger than the aviation and shipping industries combined.
So bottom line - if it's cheap and it's new, it can be considered 'fast fashion' and is probably not great for the planet.
For further information, here's 10 ways to be more sustainable28 with your wardrobe from Harpers Bazaar.
Hope that's helpful!