What does Miss Havisham want in exchange for helping Herbert?

I think Miss. H simply wants to do a bit of repentance for the way she acted. Her own increasing realisation of who she is and what she has done mirrors Pip's own greater sense of self-understanding, and also allows him to show forgiveness as he now knows that he needs forgiveness himself. Her repentance is symbolised in her willingness to help Herbert and also to help Pip and shows a reversal of her self-imposed exile from life and her family.