AFAIK, Cloudflare Tunnel (used to be called as Argo Tunnel) is free.

Argo (smart routing) is usage based.

PS: From https://www.cloudflare.com/products/argo-tunnel/, I do see "To start using Argo Tunnel, you'll need a Cloudflare plan and an Argo subscription." - I haven't enabled Argo subscription and I am able to use Tunnel. So there is some confusion.