I am the Owner of a supplier of clothing and accessories to the GA market.
Starting up a caper like this is expensive and the volumes are very low. At current volumes we have to do our manufacturing and customisation in the UK, which is great for jobs, but terrible for margins.
We have made some big/expensive mistakes along the way and found some of our market assumptions where a bit off. For instance, we had a desire to broaden the colour palette avilable for Pilots from black and navy...but all that happened was that people only bought the black and navy items!
A big lesson we have learnt is that Pilots are unlike Sailors, riders and snowboarders (wearing Crew, Joules & Fat Face respectively) in that they don't tend to want to display their hobby as an aspirational lifestyle
We are more focussed now, most of our clothing is low-key with small discrete logos and we are trying to focus on items that enhance the flying experience/safety, such as the Wristboard, Flight Cap, Gilet and trousers, but everytime we think we may turn the corner and start to turn a profit, the bank raises its charges....
We are well on our way to a small fortune, having started with a larger one!
Edited to add that in response to the above post on materials, the majority of our clothes are made from cotton