Reviewing your by-laws might be a simple matter or might have some hidden complexity. In any case, you should be able to write up a sufficient description of what you would like the lawyer to advise on and they should be able to give you a cost estimate or a fixed quote before you proceed.
Not much different from getting a few quotes from any other tradespeople. Remember, as with many things in life, you can get legal advice that is good, quick and cheap – pick any two!