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?

While it's true that you can't copyright a style, the problem I was trying to point out wasn't a copyright one.  It's that people who are familiar enough with XP to recognize the default graphics with think that it's stolen, regardless of wether or not it is.  No matter if you can prove otherwise or not that can have a negative impact on your game.

Edit: I'm talking specifically here about useing these tiles in something that is clearly not made in RPG Maker.  This would not be a problem in an RM game.

I'm not having any troubles either.  Maybe you accidentally ad blocked the source or something.

Yay!  I'm going to have to follow you on itch.io now. :)  Thanks for the update!