Link about Cache Reserve: https://developers.cloudflare.com/cache/advanced-configuration/cache-reserve/
It usually works out quite cheaply for purely HTML, JS and CSS content. But costs can skyrocket for media or attacks that aren’t caught by Cloudflare. You may also want to block certain IPv6 addresses from Cloudflare Workers that are known sources of attacks and missconfigured Nostr stuff, e.g., 2a06:98c0:3600::103 alone cost me more than all the old-school script kiddies’ botnet attacks combined.
If you do enable Cache Reserve for njump, make sure you have proper alerts in place, both for the sake of budgeting and so that you can react fast to attacks, disable fancy caching and switch to under-attack mode if you are targeted.