I'm actually going through this situation with someone that wants to commission me. So I'll illustrate this with a story:
A housewife calls a plumber to unclog a pipe. The plumber comes by, bangs on the pipe for a few minutes with his wrench, and then says:
"Okay, problem's fixed. That will be $100."
"Why do I owe you $100?" the housewife asks. "You only banged on the pipe for a few minutes!"
"Well, ma'am," the plumber replies. "The banging was only $1. Knowing where to bang on the pipe was the other $99."
When requesting any kind of service requiring a specific skill set, be it a plumber, electrician, doctor, lawyer, computer technician, etc., you are not just paying for the person's time. You are also paying for the time they invested in acquiring their knowledge.
The same goes for a cosplay commission. While you do get an end product (instead of an intangible like good health or your stuff still works), you are not paying for the end product. You are paying for the commissioner's time and knowledge that creates the end product.
For a lot of people, this is a difficult concept to wrap their heads around. But let's think about a prom dress or a wedding dress. Would you demand to get a wedding dress for less than $150? Generally not. So why is a custom-made costume any different?