CPA did not discuss rates until after bill arrived... Legal?

I'm in a weird situation.

An acquaintance, who is a CPA, offered to go over my last six years' tax returns to help me fix some errors. She offered to do this work for free at first, but two of those years were very difficult for her to check, because I had made a lot of stock trades in that time period (about 500) and she needed to review each one, which meant she had to spend a lot of time doing very dull work.

I felt sorry for her, because when she offered, she didn't know it would be such a big job. So, I offered to pay her for her services, even though I don't have much money right now, and very big debts to pay.

She said "we'll talk about that later", and instead, just kept doing the work. I brought up the option of paying her several times, and each time, she said "we'll discuss it later". We never discussed how much I should pay.

This week, she finished the work and mailed it to the tax agencies. Then, she arrived at my apartment, and handed me a bill for $3300. This bill is ridiculous in my inability to pay it, and she knows it (she knows precisely how much money I have). I was pretty angry, because she had never informed me of a price earlier, despite my efforts to discuss it with her. She told me she'd give me a break on just take $500 a month. I told her there's no way I can afford that, and told her that $300 a month will leave me with $0.00 per month for food. She agreed to $300 a month, and told me to write her 11 checks, postdated. I refused to do that, and gave her one check for $300, and told her that I'd pay her monthly, perhaps more if I have a good month (my income fluctuates, depending on business).

She was angry with this, and complained she wouldn't get paid for 11 months, and I pointed out to her that it was her that set such a high price, knowing full well I have no money.

My question is:

Is this even legal? Can a licensed CPA do work for a client without revealing how much it will cost, or her hourly rate, and just hit the client with a surprise bill, which isn't even itemized? Is $3300 really the going rate for preparing 6 tax forms? We have no written or oral contract.

I already paid her $400 a month ago for "software" she uses to do the taxes, so I'm really being asked for $3700.

I feel as though I have been set up, and am being taken advantage of.

I live in California, if this is relevent.