One too many people joked about running a Bitcoin miner... ;)

Correct, no free tier. You can use Cloudflare's free tier for all our usual stuff (caching, acceleration, SSL, etc.) that's included and add this on for minimum $5/month.

Offtopic:

Can cloudflare be used to serve purely assets? i.e say you are disqus and can cloudlfare be used to serve the javascript which sites need to include in each page?

Edit: The question was misunderstood. I asked because cloudflare ToS seems to be against serving just assets and they want to serve entire sites

Yes. Cloudflare is, among other things, a CDN, and can do that. With Workers it becomes an exceptionally powerful and configurable CDN, as we actually expose an API to control the behavior of our caching systems to your Worker.

I don't see why not. It's just like serving the assets for your own site, except they're being pulled by other sites.

I've been doing that. The problem is when users' IP get challenged by CloudFlare the assets will not load, even with security level set to essentially off.

Asked a sales rep and got told to contact support, which is pretty much impossible as I don't know who are affected until they reported the problem to us.

Have you tried turning "Browser Identity Check" off? It's in Firewall > Web Application Firewall > Browser Integrity Check.

A few times, out of desperation, I have also tried whitelisting entire countries in Firewall > IP Firewall.