Firstly, you should assess how long it takes you to draw such a thing. Estimate how long it takes for sketch, lines, colors, shades, background, etc. From there think about what it is you feel your time is worth to you when you consider the prices given.

As for my recommended breakdown of the examples given, and based off similar works I've seen and commissioned $150-$200 range.

Single character Full body + shades = $120-$150
Background (depending on complexity) = $30 - $50

A few other factors to consider are your country of origin and living expenses. Make sure you're not underselling yourself.

Thank you, that makes sense. Useful, really.
It's actually a bit of a challenge for me to estimate time, because I only draw for fun and only when I'm not busy with other things. I'm not a full-time freelance artist and I have IRL work.

Understandable. But if you can, try and track like 4 hours this night, 2 hours the next, just give yourself a ballpark idea. And if you're just doing it on the side as a fun way to make a little, it's ultimately up to you to decide what your time is worth to you. If you don't mind working for a little, then charge a little.

Happy to gave helped however it is I did.