Almost impossible to give a general range. It will depend on scope of project, requirements,area skill level of the artist. Your best bet would be to put together all the specs and requirements and get several some quotes. Concept art in particular has a wide range depending on needs/use. Since often the goal of concept art is a back and forth to determine direction, just commissioning concepts can be tricky, since ultimately you are actually paying for ideas and not really finished pieces. Also concept ranges from very rough exploratory ideas to nearly finalized direction.
Working with a concept artist(s) works best when you have good relationship and know what each other expects. Best to small very small with someone new, and determine if it will work out. It is a different working relationship than with commissioning final art.
Knowing what you want in the end will help the artist give you an accurate estimate.
Will depend on the artist. Typically professionals won’t, but you may find someone that will. Often artists in school/new/hobbiests will be more willing to do something like that, but even then it will probably depend on whether or not they believe in your idea/project. Best bet, just ask.
First make sure that you are providing them with as much detail as they need to execute what you want. Part of what an artist does (concept or production) is fill in details. If you give them enough details, there shouldn’t be too many surprises.
Check their portfolio, ensure that have expereince in providing you what you are asking them to do.
And most importantly, start small. At first just one task at a time. That way if for any reason it isn’t working out, you haven’t committed too much time or expense. This also benefits the artist, sometimes people just don’t work together well, regardless of skill/knowledge.
As much as possible. More detail is better than less (at least at first). The details of the game and how the assets will be used (for final assets). For concepting, anything you feel is relevant to what you want to see in the end. Existing art/designs that show the direction you are going for.