I would say any price, depending on the adoptable... I see some people who charge around 7-10$ but it includes the full-body picture (the adoptable) and a nice colored or lineart of a headshot of the character.
Never more than two dollars, and even that's pushing it for me. Suere i might miss out on some cool designs, but i have pride myself to be able to make my own great original designs. And Commisions to draw a charcter i've designed however, is another story. I'd pay a lot to have someone draw something of my characters to put on my wall.
moi je met encore 5€ et 25 ou même 30€ après comme disent les autres, ça dépend du design si j'ai un coup de cœur (ce qui est très rare, la preuve j'ai jamais adopté un perso) je pourrais mettre 20€ ou plus sans hésiter.