When I got mine I paid $20 for her MIB and I was even lucky enough to find an extremely nice seller who was kind enough to cover the shipping cost for me! So really it depends on how much you feel comfortable selling her for, but I would say you can safely ask anywhere from $20-$35 for her MIB and that is without shipping costs factored in.
I hope the person you sell her to loves her! I know that I adore mine to pieces. =3