Citations to sources that do not exist
Models cite [7] when you provided three sources. It is a specific and common failure, it is trivially detectable, and it usually reaches production because nobody checks.
An out-of-range citation should fail closed in your application: show the claim without a link rather than a broken one, and log it as a quality signal.
The unused sources tell you about retrieval
If you retrieve eight chunks and the answer cites two, six chunks were paid for and discarded. That is a retrieval problem showing up as a cost problem.
Consistently low usage means k is too high or your ranking is weak. Reducing k or adding a reranker usually improves both the bill and the answer.