> If a page is static, then CloudFlare can cache it. But if you set your cache headers appropriately, and use efficient serving code like nginx, I imagine serving static content is pretty darn cheap.

With the static content it's not the cost of serving it, it's the fact that Cloudflare is serving it from a large bunch of distributed servers that are likely to offer far lower latency to the end-user than your servers. With modern web pages often containing hundreds of objects, this can make a big difference to page load times.

If all your customers are in one geography this is less of an issue, but if you have a global audience this can make a huge difference.