These are among the reasons that I refuse to make my kids costumes almost without exception. Usually people are surprised by this due to the fact that I love to sew and craft so let me break it down for you:
Just about any seamstress knows that these days, you do not save money by making your own clothing--especially children's clothing. If you are purchasing your materials retail, you usually spend way more than the cost of a ready made garment before you even put in the time and effort to make it. Costumes are no exception. Take this Cinderella costume for example:
*41/2 yds blue polyester satin fabric from fabric.com--$22.41. (plus more for shipping)
*notions (ribbon, interfacing, velcro etc) 10-20$
*Total cost: $42.92-52.92 depending on how crazy you go with the notions, and this isn't even including extras like a crown or gloves or shoes.
I found a much cuter one on ebay for under $30 dollars brand new including gloves and shipping
Last year I did actually make Greta's Tinkerbell Costume, it consisted of me hunting down a leotard and slippers, bleaching the pink out of them and then hand dying them green. Then I had to cut a ton of green tulle strips and tie them on an elastic waistband, and make pom-poms out of yarn for her slippers. On top of all of that, I had to purchase wings.
Through the night, Greta had a potty accident in her leotard, ripped a hole in her tights and ruined her slippers beyond repair. So all of that money and work went down the toilet so to speak.
I think many people make the mistake of thinking that a halloween costume should be easier to sew than a regular garment, and this is simply not the case. I guess technically you could get away with not finishing your seams or pressing, but the general construction is still the same.
I'd rather spend my time making something my daughter or sons would use over and over until they grow out of it--or better yet--something for me that I wouldn't grow out of (hopefully).
Hopefully I've convinced you to relieve yourself of the Halloween pressure and just purchase the cheapest thing you can for your kids this year, and then spend all of the time you would have on costumes for your handmade Christmas gifts (wink wink).