Firstly, i hate it when some thing like this happens to me. its really frustrating and can get you really angry.
What i would do in your situation is let her vent off all her anger, then i would offer to stop work immediately and not charge her a penny for the work done. say it was an honest misunderstanding between the two of you and you dont want any client of yours to feel like they've been misled.
if she wants you to continue work then, offer to complete the outsides at the cost agreed initiallly, or offer to give her a quote to do the insides at your normal price.
by offerring to pack up and leave without a penny, she cant accuse you of pulling a fast one. just make sure you do oit with a smile and in a relaxed manner.
the good thing is that these people are few and far between. personally when they start behaving like that, i dont want them anywhere near my business so loosing out on the job is a small price to pay.
good luck!