Illustrations are an excellent to get a message through in a more comprehensive and memorable way than just plain text. Great for any kind of publication; print or digital.
The price may vary depending on the size and complexity:
Spot illustration: usually composed of few elements, works well without a background, elements may not be situated in any specific context. Great to complement a blog or article, to create a piece of content for your social media, for specific marketing or communication campaigns, to complement your brand image...
Page illustration: tells more of a story on its own, may even have characters, but for sure it has a specific context, even if it is not a real one. It works well for complementing a text but it can also stand alone and express a complete message.