The rate is whatever you negotiate.

Everyone brings their innate skills and experience. Some people are really excellent artists, some people are not. Some people can produce quality results very quickly, other cannot. Some people have decades of experience, some people are beginners. Since it is contract work, some people bring many excellent tools, some people have no tools.

Exactly how much you want to pay is up to you. You will get art on par with what you pay.

Depending on the number and quality of sprites, the time frame, and the number of iterations I can imagine this paying $100 or $300 to an art student for a few simple sprite sheets, ranging to $100,000+ to a professional art studio for many tens of thousand sprites.

There's not really "standard case" in art and finances. People have different skills and different goals. Others make it for living, others try to, some do it for fun.

- A pro art studio that is licensed company paying all kinds of bureaucracies, taxes, rent, software, personnel costs etc could make you a very costly offer.

- A freelance graphics designer that is trying to make a living out of it would be cheaper but close to hourly salary around where he happens to live, which still varies widely.

- A hobbyist/student just making art for fun might make them even for free if he has spare time, is interested in your project or wants some portfolio material, for example.

Quality and prices might vary widely even within each of these cases. You post your offer somewhere and wait for offers and compare them against respective portfolios. You might find some offers that seem good and even some people that you want to work with.

Remember if you can't be accurate with all the specifications of your assets you are likely get an per-hour prices only that might not tell you much about the total price. You could get a lot of personal messages asking you about this and that while others are already negotiating deals with you. So always try to give as much information about what exactly you need (pixel sizes, palette and visual style with examples, possible animation/angle variations) to make it less of a hassle for everyone. Try to get to know the artist and agree on a fixed price per asset, that makes things less awkward and more swift.