Blastr is very efficient. Both because #[4]​ did a great job with it but also because it only sends events.

Where you choose to host your relay and what implementation you choose to run will impact your costs. You can run a very lean private relay on a $10 VPS.

If you run a large relay, your bandwidth consumption will be substantial across all implementations. Picking a provider that allows for unmetered or has a high flat rate limit is imperative to control cost.

Taking it an extra step, if your users are going to be spread out geographically and performance matters, you’ll want to provide regional mirrors. Keeping these mirrors in sync and putting them behind a load balancer has additional costs.

The upside to cost doesn’t really have limits but there are low barriers to entry.