In "The Crucible" what does the quote by Parris, "Oh, Abigail, what proper payment for my charity! Now I am undone!" mean?

Parris utters this phrase after it seems like there was some forbidden thing going on in the forest between all of the girls.  The first scene opens up with him questioning Abby about what happened the night before, as he discovered them dancing in the woods.  She denies having done anything but dance, but he is worried that there might be more.  There are rumors of witchcraft floating about, and Reverend Parris is worried that his reputation as a minister of the church might be tarnished.  How would it look to have a minister, who is supposedly called of God, be associated with witchcraft?  His niece (Abby) and his daugher (Betty) being linked to doing evil things in the woods would tarnish his reputation as a minister.  He is already worried about his reputation, because, as he states to Abby, there are already people out there who are "sworn to drive [him] from [his] pulpit."  People don't like Reverend Parris, and there are groups and factions out there who want him gone.  If the witchcraft rumor holds any water, his enemies can use it against him to get rid of him.

So, he is worried and frustrated.  As Mrs. Putnam confesses she sent Tituba out with the girls to "conjure spirits" to find out who killed her dead babies, the witchcraft angle becomes more solid, and Parris feels like there is no hope for him.  He realizes that Abby was lying about there being nothing more than dancing going on, and asks her, "Then you were conjuring spirits last night."  Abby tries to deny involvement, and blames it on "Tituba and Ruth."  But, that isn't a clean enough break from the taint, and Parris utters the above phrase.

That is the background to the phrase, and it means, in translation, that he feels that Abby, who he took in and cared for after her parents died, has been very ungrateful.  She has not behaved properly--she should have been a good example and lived cleanly in order to pay her uncle back for his kind service.  He feels that her slip-up will be his undoing as a minister in the town.  I hope that those thoughts clarify the meaning and background for you--good luck!