Commission tips?

I’d like to try my hand at commission work. I decided my base price willbe $7 dollars for solo portraits. But I’m still trying to hash out other details. So I came here to ask for reasonable suggestions for price given my current skill.
 
 
Here is my gallery

It depends on how long it takes you to make each one, really. But maybe you could do this:
 
$7 for solo.  
Add $3 for every extra character.  
Add $2 for human.
 
$5 for portrait.
 
$1 for single colour background.  
$3 for more detailed background.
 
IDK. I’ve never priced commissions before.

Hey that’s a good start.