Got it. This does make your side seem more excusable, however only in detail; I'm pretty sure you're aware you do likely owe the funds, so my recommendation would be to offer a payment plan, something approaching $100/mo.?
This would do several things:
Demonstrate your willingness to pay, and within a reasonable amount of time.
Allow you to spread the payments, thus ensuring it's less of a burden.
If she turns this down, she'll likely take you to small claims, but there you could likely prove said willingness to pay. You may also point to the fact that the amount due was not incurred all in one lump sum, and you are asking for the same type of schedule.
Also, does your lease state specific amounts and did you notice other differences or fluctuations (due to agreed/other rent/fee changes?)