You can save a lot of money on Halloween costumes if you think about it a little bit. Usually, the first week of November the stores will mark down the Halloween costumes with deep discounts. This is your time to strike.
Get together with your spouse and your children and see what kind of costumes they might like for the following year. This can sometimes be difficult to guess exactly, but at least you’ll be putting down less money for these costumes. You will need to take into account another year of growth for each of your children and buy them a costume in the size it will likely be in a year’s time.
I’ve gotten very lucky but every year my wife has estimated the size our children would be next year, she has gotten it right. Stores all around will likely want to get rid of their Halloween stuff so that they can start putting in Christmas displays, so their impatience is your gain. If you do it this way, you should never have to pay full price for a Halloween costume.