Locking onto a set price for a commission is hard to formulate, considering each outfit is different and may require more time to make. But for the most part commissioning in general is you paying for the time and effort spent along with quality of a piece. Asking what's a reasonable price for a commission is too vague of a question.
It really depends what you're having commissioned.
Honestly-- if I'm commissioning someone I firmly believe for the most part that I am paying that person basically for their time that they spend making it more than anything else. I shouldn't expect them to cut me a break just because I could make the same thing for cheaper. And if that was the case-- I would probably do it myself then.
Btw, I do comissions on the side. I haven't charged anyone more than $125 for a costume. I don't see a reason to charge someone more than what they would pay for one online.
Again, this really depends. Considering in some instances I believe there is a very valid reason as to why some people who commission overprice some of their work opposed to retail made cosplays. Depending on the time they spent, the materials they've made the cosplay out of, the details they've put into it and the skill it took the make it. It's the only way people actually make a profit out of their work.