To expand on Cyangmou's post:
In practice, artists usually charge per asset or per batch, so that the price is clear up front to both parties, but they use their desired hourly rate and the expected time to completion to calculate that price. However, more experienced artists will still generally have higher prices than less-experienced artists even despite usually being faster, because the art they produce is higher quality and because they're more versatile.
If you can't provide a well-defined end to the work or if you're very inconsistent about what you want, making it difficult to estimate time to completion, then expect to pay an hourly rate. Some artists also charge hourly for revisions, as those are difficult to estimate because they depend on the client. It is in your best interest to lay out what you want very clearly and completely, provide visual examples, etc, so that the artist can figure out what exactly you want and create it right the first time.
Seconding Cyangmou about approaching artists with an offer. Artists hate giving quotes.
Since it sounds like you're trying to figure out what your budget should be in the first place, then approach artists with your full asset list and information about your desired style (visual examples always help), and ask them for estimates for various parts of it. After you get an estimate, you could even ask them how they calculated it (just make sure it's clear that you're just curious and don't intend to debate their prices), so that you get an idea of how much work those assets are. As you work with more artists over your career, you'll find yourself being able to do this about as well as artists are, and you'll be able to budget early.
I mentioned asking artists for quotes on various parts of your asset list because many artists can't do everything, or lack the confidence to tackle it all. Artists will often quote higher prices for things they're not comfortable with, because they expect to need more time to get it right. Letting them quote things separately and allowing them to not do some parts of the work will get you the best deal for the work they can do well. For the remaining work, you can hire another artist, or even ask the same artist again after they've finished their part of the work, they might feel more confident about it by then, because they'll have a better idea of what you and your desires are like.