I generally work in graphite pencil on an A3 page in portraits (shoulders and head).
$20 base rate with $10 per hour of drawing, comes to approximately $80. This is an estimate, drawings may take significantly longer.
However, all is negotiable. I may be willing to draw anything you wish in whatever size and medium you like as long as ample references are provided. Prices may also be discussed.
I would ask an allowance of one month for any commissions, time and inspiration permitting.