I'd need to do some research ingesting events into neo4j. It shouldn't be much more than running a relay in terms of storage space. Where you'll pay the most is in CPUs. On Amazon, the difference between 2 and 4 CPUs looks like $50/mo. I'm sure you could have good performance on a small or medium sized web of trust in their medium 2CPU/4GB instances for $15-20/mo.

For all of Nostr, primal style centralized cache with lots of users, I'd guess you could spend $50-100 or more a month.