A small number multiplied twice
Forty tokens of metadata looks trivial. Multiply by eight chunks per query and by two hundred thousand queries a month and it is a real line on the bill, for information the model mostly does not use.
The URL field is usually the worst offender: it is often the longest value and, unless the model is quoting links back to the user, it is paid for and ignored.
JSON wrapper against a header line
Wrapping each chunk in a JSON object repeats every key on every chunk of every query. A plain header line — source, section, date — conveys the same thing at roughly a third of the cost and reads just as clearly to the model.
The cheapest option is sending no metadata at all and citing by index instead, resolving the number to a source in your own application. That works whenever the model does not need to reason about provenance.