It is annoying that the answer is basically "we paid someone [Cloudflare] to do it." In light of that I decided to run some of my own numbers and figure it out. They are pushing about 245 Mbps out of Cloudflare (averaged over the month). Wholesale IP transit prices are anywhere from 10 cents to a dollar per Mbps depending on volume. Cloudflare dumps 40% of their traffic over peering, putting their price at $14.70/mo.…
Not even close to comparable. I think you're ignoring all of the functions they're paying for. They're not just hosting a few files. Also, your "worst case" is literally the most optimistic best case for a standalone single server, which completely disregards any redundancy and assumes that the $30/month server is truly unlimited/unmetered in every way. That's not a good assumption.
Cloudflare is a global CDN that will be fast for everyone regardless of location and will soak up bursts of traffic with ease. It's also fast in a location-independent way, which won't be true for a single server in a single datacenter somewhere.
Finally, the amount of effort it takes to do this with Cloudflare is trivial. The amount of effort it would take to maintain and optimize a standalone solution is not negligible.
Cloudflare seems like a very good deal to me, unless you value your time at $0 and you have access to these truly unlimited, high-performance $30/month servers.