Hoi An is famous for tailoring, which is immediately evident by the number of tailors available in the Ancient Town.
With so many tailors, deciding which one to go with is difficult. Yaly Couture, a well-established tailor with three locations, is a safe way to go.
Honestly, prices will differ depending on who you are dealing with and how good your skills of negotiation are.
It's best to go in with a picture of what you want made, or at least an idea. They have books of shirts, pants, dresses, etc., but there are so many things to search through that it'll be impossible to make a decision.
Make sure to adjust your expectations. While the tailors are very talented, they aren't going to be able to make an exact replica of what you want. Also, make sure to speak up or forever hold your peace about fit. If you don't like the way something fits or want something adjusted, make sure you tell them as soon as possible.