Rates according to hours, I try to make a livable wage!
Your own financial standard should guide you more than requiring to meet a certain standard of mine! So just send an offer based on your frames of reference!
Character sketches can be done 1-2 characters per day, 1 with refined lines and coloring. See Ixts character collage (1st pic).
Color sketches typically ½ - 1 day or 1-3 with background depending on complexity! See hbd hoooook and Enthusiasm midstage (2nd, 3rd pic).
2 days for initial paint, 3 days for polish!
Full paintings, the examples such as Enthusiasm, Pomegranate and beer and Chinese fantasy pieces are 1½ - 3 week works at full time work hours. See Chinese fantasy pieces, Pomegranate and beer and Enthusiasm (4th, 5th pics above and Gallery Highlight (Twi'lek one)).