I'm sort of just echoing what others have said, but yeah, keep in mind how much more there is to a commission than the materials.
- They've probably spent months if not years of their life and hundreds of dollars gaining the skills, knowledge and tools. They've spent anywhere from $70 to over $1,000 on a sewing machine alone, plus all the materials they had to buy to make costumes to learn to sew, plus things like scissors, irons and ironing boards, rulers and measuring tapes, machine accessories like presser feet and bobbins, machine maintenance, etc. plus several hundred dollars if they have a serger to professionally finish seams.
- They'll spend time communicating with you, doing research so your costume looks authentic to the character, and finding and purchasing materials, on top of the hours it takes to actually construct the costume.
- They have office/living expenses required to get you your costume. Electricity, an internet connection, a car they buy gas and insurance for that they use to go purchase supplies, etc.
Most seamstresses I know don't even make minimum wage, let alone what their time and skills are actually worth. So it's fine to tell someone you can't afford their work, but when you can make minimum wage flipping burgers or manning a cash register after a day of training, don't begrudge them what they charge.
There are probably some out there who do overcharge, but I don't think it's common.