How many $400 Sheitels do you think a Macher sells per week? Of course they dont make $20 per Sheitel, but they dont sell many of them either
You do realize that selling sheitels is just a part of their business, right? They also wash and style sheitels. (Just like the hair on your head, sheitels get dirty and need to be washed.)
Some also cut hair.
Some also sell hair accessories.
And a lot of sheitels cost a lost more than $400.