An illustrator early in their freelance career, building a portfolio and taking on a mix of paid and lower-budget work, often lands in the $15-25/hr range. Illustrators with an established style and steady client base typically land closer to the $30-40/hr median. Illustrators with a highly distinctive, in-demand style, or those doing licensed commercial work for major brands, can charge $50-65/hr or more, and often price per piece or per project rather than hourly at that level.
A rate only matters if it actually gets collected. Once you've picked a number, send a professional invoice that states your terms clearly, and consider a simple contract for anything beyond a quick one-off, it's the easiest way to avoid scope creep eating into that hourly number. If you want to see how your rate translates into real take-home pay after taxes and time off, try the freelance take-home pay calculator.