Pretty much what everybody else said.
"A simple shirt" means different things to different people. What kind of shirt is simple? A t-shirt with something painted on it? A knit shirt with a unique collar or shape? A button-down shirt? A button-down with peculiar collar and cuff configurations that can't be found at a thrift store?
It does help to know how garments go together and what your own costs on your own costume are, because then you can judge whether you're being fairly charged for the work. But you're not just paying for the materials to make the shirt, you're paying the commissioner's labor and overhead as well. Now, if it's a commissioner who's only doing it for a little cash on the side, nothing complicated, perhaps they don't value their time that highly and lowball the price for a fast sell. But when you're talking about commissioners who do this as a full-time job, there is a LOT factored into their labor prices that will naturally skyrocket a price much higher than the bootleg costumes sold online from overseas shops.
Think of it this way: it's the difference between going to Krispy Kreme to get a dozen doughnuts that are churned out by a machine, versus a dozen custom-decorated gluten-free red velvet cupcakes from a local bakery that baked them this morning. Of course the mass-produced doughnuts are going to cost less. Commissions are artisan work, they're not mass-produced. If you don't want to pay the price for hand-made work, then by all means buy it online or do it yourself.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com