What routing a message actually costs
No price is quoted on this page, deliberately. Solidus's own live surfaces currently disagree about pricing by a factor of fifty on a different product, that conflict is unresolved, and relay has no published rate card at all. Quoting a number here would be inventing one.
What follows is the cost structure, which is real and useful even without a price attached to it.
What a mediator actually spends
Storage, and it is time-shaped. A queued message occupies space from arrival until collection, or until retention expires. Ours caps envelopes at 1 MiB and holds queued messages 14 days by default. The cost driver is therefore not "messages" but message-days: a million small messages collected in seconds cost less than a thousand large ones sitting for a fortnight. See queue depth and backpressure.
Bandwidth, paid twice. Every message is transferred in and out. Nothing exotic; it just means the unit is bytes moved rather than messages handled.
Resolution. Routing a did:solidus means an RPC call to the chain. Ours caches, and a cached
answer keeps its original block and timestamp rather than being re-stamped fresh, see
chain-anchored routing resolution. Caching is a cost
decision as much as a latency one.
Admission checks. A credential-gated grant calls out to a verifier. That has a real cost per admission, which is one reason admission is not checked per message.
The operational floor. Availability, on-call, upgrades, monitoring. This dominates at low volume and is why a mediator serving ten agents costs roughly what one serving ten thousand costs.
Why per-message pricing describes this badly
The dominant costs are time-in-queue and a fixed operational floor. Per-message pricing matches neither: it overcharges a chatty agent that collects instantly and undercharges a quiet one that leaves large envelopes sitting for two weeks.
We are saying that rather than quietly picking the metric that flatters us. When relay does publish pricing, judge it against this page, if the metric has no relationship to the costs described here, ask why.
The honest comparison
Against self-hosting, the question is whether your volume clears the operational floor you would otherwise pay yourself, a server, plus the on-call nobody budgets for. See hosted vs self-hosted and vs running your own.
Against doing nothing, note that a mediator is only worth any price if your agents genuinely cannot be online together. If they can, deliver directly, when you do not need a mediator.
What you should not read into this page
No modelled price, no projection, no unit economics you could put in a spreadsheet. Those exist elsewhere in Solidus material for other products and are labelled modelled; nothing about relay has been through that exercise, and presenting a cost structure as though it were a price would be exactly the drift this estate is written to avoid.
Nothing here is a privacy claim either, what a mediator observes while doing all of the above is what a mediator can and cannot see.
Relay implements Coordinate Mediation 2.0 and Pickup 3.0; no independent DIDComm conformance suite has been run, so "implements" is the claim and "conformant" is not.