It depends on your budget. Define what budget you have and go from there.
Commissioning a talented person to make one for you will be the best quality - but it can be expensive - and the better they are the more they can charge. I would expect to pay over $500 for most costumes, and that won't include wigs and such.
I was quoted $3000 for a costume once, and $800 for a certain jacket. These were fair prices for what they were - however, for something less complicated and going with a less skilled worker would have cost me less.
I didn't buy either of those - I wasn't even looking for a quote, it was just part of a discussion. The fabric alone for that, though, cost me $300 just for one of the materials. So you have to consider the costume itself and how expensive it will be to make + labour - and people who are really good and are doing this for a living are going to be charging ~$30 an hour or more.
I started sewing because I couldn't afford to have someone else make it for me and wasn't willing to put up with poor quality. My Legend of Zelda costume is costing me about $300. Now, if I was gonna charge someone for me to make it, they'd have to pay $300 for those materials plus me for my time. It would probably end up being over $1000 because chainmail takes a long time to make. $700 would cover about one work week - about 35 hours at $20/hr (what I would personally charge), however, I'm thinking the chainmail alone will take me 8+ hours and I'm only making sleeves - so 35 hours for total work might be cutting it close.
I just want to give you an idea of costs and where they are coming from if you decide to commission someone. The fabric itself will be the lower cost between it and labour, usually.
Another example is that a real, professionally made replica for a coat from Doctor Who is going to run you over $1000 for just the coat. It's probably over 40 hours of work and the guy had to develop the pattern himself. Just as one example. There are cheaper mass-produced ones for like, $4-500 and the really cheap ones form China for like, $200-ish. The fabric for the professional replica is probably over $100 alone, though - to give you an idea of the differences in quality of even just the fabric.
I hope that gives you an idea of costs and the choices you will have to make.