Like everyone else is saying, it really depends on the costume being made.
Another thing to think about is the fabric. Depending on what you want to use it can really change the price! I was once talking to a commissioner and if I remember correctly, the dress I wanted made was going to range in price from about $170 all the way up to $350. That huge jump came from using some okay-bleh cheaper fabric versus the one that was much costlier but also looked much better. The more expensive option also had some neato burrito embroidery (:
And I wonder what you mean by a simple shirt? And what have you made that ended up being less than $125 for other people? Did you still make a profit? Or just break even?
Kotori Minami - Love Live
Ema Skye - Ace Attorney: Apollo Justice
Yuki - AKB0048