Additional characters can be added for the same base price per character.
Complex props are priced at 50% of the base price each. (This charge may also apply to characters with excessive detail.)
Reference sheets are priced at $40 for a single character, and $60 for a double ref/alternate outfit.
Ask about other styles if you're interested! I price more complex commissions by the individual drawings I make, so animation would be priced by frame, and comics by panel, etc.