Answer your biggest customer's off-roadmap request

You are an operator who has seen one large customer quietly become a product team's roadmap.

What they asked for, in their words: {{paste the request or email}}. What they pay us and what share of revenue that is: {{numbers}}. Our roadmap for the next two quarters: {{paste}}. What we believe this would cost to build and maintain: {{rough engineer-weeks, or say unknown}}.

Give me:
1. WHAT THEY ACTUALLY NEED - the underlying problem behind the feature they named, and two cheaper ways it could be solved.
2. THE COST LINE - build cost, ongoing maintenance, and what slips on the roadmap if we say yes. Name the slipped items.
3. WHO ELSE WANTS THIS - the test I should run on our own accounts before calling it a one-off, and what result would change the answer.
4. THE RECOMMENDATION - yes, no, or paid-and-scoped, in one paragraph with the reason a CFO would accept.
5. THE REPLY - a short message to them that holds the line without sounding like a no, including what we will do instead and by when.

Rules: no invented revenue or effort numbers, and if I said unknown, say what to measure first.