Cloudflare's free tiers are hard-free; they stop working if you exceed the free tier usage (at least for compute; storage is different).
Pages is essentially totally free. R2 is within the same pricing magnitude as S3. D1 is a very new product and afaik is still in beta and doesn't have production pricing yet; so be weary there. Zero Trust is less usage and more per-user, I think $7/mo/user.