There are industry standards for illustration rates, but they both vary widely and are notoriously opaque. In China in 2020, I know the industry rates are:
Interior book illustration RMB 600 – 1,000 (USD 85 – 140, GBP 68 – 114)
Book cover illustration RMB 3,000 – 4,000 (USD 424 – 565, GBP 342 – 456)
Working out your own rates
First thing, work out your minimum fee per hour of work.
What are all your costs (rent, food, transport etc), and add on top of that a decent annual income. Now break it down to cost per five day week, per eight hour day, per hour – and that’s it, your hourly wage.
How long will the illustration commission take? You can work out your bottom line easily now.
Set fees. Most places that use images will have set fees that are fairly standard across the industry. It’s hard to track down reliable figures, but in the UK something like this.
If your illustrations are going to be published by a magazine, for example, ask up front exactly how they are going to be used (what size and where). It’s unlikely you’ll have much room to negotiate, but at least you’ll know if it’s acceptable to you (see step one above), if it’s inline with whatever the going rate is (as far as you can pin that down) and if you work for them again you’ll have a point of reference.
Use of image. Magazines, with set circulation numbers and areas of distribution, are fairly easy. Anything else and it’s hard to get a handle on, but have a look at Getty Images, click on an image, and then choose the “View Pricing” link. You’ll get a seemingly endless list of pull down menus – is the publication print or web, how many readers, what countries is the publication available in, what proportion of a page does the image take up, on and on. Their pricing varies dramatically according to your use of the image.