When I buy a garment, what portion of the cost goes to the workers?
Brands are notoriously secretive about what percentage of their retail prices go to the worker, so there is very little we can say for sure. However, while labour costs vary, for most garments, wages for production will scarcely exceed 3% of the price you pay in the shop.
This percentage could be drastically less for an item from a luxury fashion brand, since workers producing high end garments do not typically earn more than workers producing for high street brands.
Because this amount is so low, paying workers a wage they can actually live a decent life on should not be a costly affair.
For an example of what a price build-up of a fast fashion brand could look like, look at this calculation for a Zara hoodie.