It is my understanding that cavities are caused by a bacteria, and that having untreated cavities in the mouth makes it more likely that a child will develop more cavities. She may have more cavities that you can't see.
Talk to the dentist ahead of time about a payment plan. If you are up front and honest, you may be able to work something out so that you can get the work done now. You could also consider getting the work done at a dental college, as it would be far, far less expensive.
Some other ideas....
Use floride toothpaste. (I know some people on this board disagree). We used to use toothpaste without floride and one of my children got 4 cavities and the other got 2. We switched for floride toothpaste and haven't had any cavities since.
Brush several times a day. Ideally, brushing after every meal and snack. We just brush twice a day, after breakfast and before bed time. My dentist said to try and brush as quickly after eating food or drinking juice as possible, as the worse damage is done in the first 20 minutes following the food (or drink).
Limit how often your child can eat or drink anything other than water. One habit that my dentist said was wrecking havoc on my kids teeth was little them sip on juice. They didn't drink that much juice over the day, but they drank if very slowly. Now they can't have anything between meals (and planned snacks) except water. Darn near everything we eat breaks down into sugar once it is in our mouths, so munching, even on healthy foods like fresh fruit and whole wheat bread, is bad for our teeth. Even if a child has just one sip of juice or just one bite of toast, their mouth is cavity heaven for 20 minutes
Hope this helps.