I am 58 female w/ one small cavity upper back tooth and one filling lower back tooth w/ small cavity bottom side of filling..? Is do I go composite or cerac? Price diff of 800+ per tooth

Size Matters. Depending on the size of the cavity and how far is extends across the different sides of the tooth, a Cerec restoration could be worth the investment. When a dentist recommends a composite resin filling, it is usually the best treatment for that tooth to restore it to optimal form and function. We budget is a concern, sometimes a dentist will offer choices. Ask your dentist about each tooth.
More info needed... The answer to your question is not cut and dry, black or white. Determining the best restoration is sometimes a matter of preference... there may not be an absolute right or wrong answer. If you doubt the diagnosis, get a second opinion from another dentist (who can actually examine the teeth in question).
Hard to answer. Complex question. Pros and cons. Best bet discuss pros and cons with dentist. Consider a second opinion before costly treatment.