The token overhead of forcing JSON / structured responses.
The token cost of JSON mode is small next to the cost of parsing flaky free-text output. Keep schemas lean and it's a bargain. Estimate the whole call on the LLM token cost calculator.
Free structured-output cost calculator — JSON schema and formatting add tokens; see what structured responses cost.
A little. You send the JSON schema as extra input tokens, and structured responses tend to be more verbose than free text. On high volume that adds up, though it usually pays for itself in fewer parsing errors.
Almost always — reliable, parseable output saves engineering time and failed requests. Just keep the schema minimal and don't request fields you won't use.