Depends on the size of the game of course. Between 200 and 900$ is rough estimate for an RPG sorta game, I would say. Pixel art would probably be the cheapest.

RPG OST's can get very large, so if you don't find a good deal such an ordeal can cost several thousand dollars.

Well for 3d art for a decent artist just starting out is about 15.00<= very low end if you get lucky to 20.00 an hour

As for an astablised artist starting rates are 25 and up from there. Keeping in mind that's the industry standard for a very good free lance artist

A high end 3d artist like someone from a well know company with high end title's under there belt it's going to be a lot higher

Best thing to do is go to a site like polycount an ask or see for your self. (freelance section) But beware if you post a job there and try low balling you may get a very unwelcomed response.

They take there job very seriously! Like most artistic people, that work for a living, they put there (heart and soul) into there craft.

Ps  Location is also an issue, if you are able to hire some one where wage's are lower it will be cheaper also. But that go's with out saying.

Though just like Spring mentioned, RPG tiles tend to be very expensive due to the need for so many different tiles. Platformer like games like megaman like or mario like can be really cheap though. It all depends on the style and level of detale youre after.
for a NES like $900 for an RPG would be about average.
but 3D or Snes like an the price can get into the thousands.
Vector depends on the level of detail but it can go as low as $300.
Megaman like / mario like pixel tiles also around $300.
But once again level of detial realy drives the price.

If youre looking for a cheaper alternative for tile artist for snes/nes or general pixel art. Send me a PM and we can talk about a deal. Im always looking for an excuse to draw some tiles.

The cost is going to depend on a lot of factors, what the subject is, how big the resulting set should be, what the style is, if you pick a good artist, and more.  Subject is the biggest concern.  As mentioned before, a platformer is less work than an RPG.  I would put RPG on the expensive side of the list.  I don't think people really understand just how huge even a basic RPG set can be.  Something inbetween might be a overhead shooter, since you're not having to do as many effects or locations.  Puzzle game would be the cheapest, just some menus, some backgrounds, a cursor, and a few blocks or whatever.

Your second concern, the style, is really huge.  The cost difference between a simple style like Earthbound or one of the early Dragon Quest games would cost much less than something more involved like Seiken Densetsu 3 or Alundra.  Simple doesn't have to feel cheap or cartoony though.  Simple can also be dramatic and atmospheric depending on how you go about it.  Limbo, for example.

The third biggest concern is scope.  This is pretty difficult to pin down because just simplifying it to a few locations might not be enough.  Even something as simple as a single house can get very expensive if you want to have every bell and whistle.  Or, since it's a house, every lamp and tea set.

I highly recommend figuring out exactly what you want out of this project and why so you can get your money's worth.  Don't leave it up to the artist, you can easily end up paying for things you don't actually want that way.

So, for the artist cost, you don't always want to go with the cheapest.  Cheap can be good, but most of the time it means inexperienced.  An experienced artist is going to be much more per hour but they'll also work dramatically faster, have to redo less, be better at figuring out what you want and getting it to you than someone who's new will.  I recommend going for someone who can either do the style you want well or who has a really solid and diverse portfolio which means they're more likely to be able to pick up a new style for the project fairly easily.

FYI Pixel Art is only cheaper when the tiles are small (16x16 is best in my opinion) and the style is relatively simple.  It gets vastly more expensive very quickly the bigger the canvas is, and the artwork gets easier to do with other methods.  Vector doesn't care about size at all, but gets more expensive the more detailed you get pretty quickly, due to the constraints of the medium.  For high def with a smaller scope digital painting is your best bet, it's the easiest to make for a wide variety of project sizes and styles.  3D is best for bigger projects, because the set up is the expensive part.  Once you've got the basics down you can make small changes to make "new" objects and the costs start to offset.  I wouldn't recommend it for a project like this.

Since this project is still pretty hypothetical, why not consider taking an existing set of resources that you like and hiring someone to expand on them?

nice reply detailed and informative