Shall the message fee include cost of parachain finality transaction?

[svyatonik]

When we are delivering messages between two relay (or standalone) chains, then we only deal with GRANDPA finality. GRANDPA finality transactions have some cost. To cover these costs, we have two things:

1. some GRANDPA finality transactions are free - when they're delivering mandatory headers;
2. there's some additional relayer interest in the message fee, which may (at least partially) cover this cost.

Initial plan for Kusama<>Polkadot relay<>relay bridge was to run only-mandatory-headers finality relay. It means delivery lags, but the cost of running this bridge is zero.

Now we're dealing with parachain <> parachain bridge. It requires additional parachain finality transaction in the pipeline. Shall we add this cost to the message fee? Shall we just increase relayer interest? Shall we make some parachain finality transactions free (Pays::No) - e.g. there's one free transaction every 1024 relay blocks?

We shall think about that after #1318

[svyatonik]

The final fee idea is that it'll be taken at the source parachain (i.e. not at the bridge hub). We only charge relayers for bridge-hub transactions and then reward them from source/target parachains sovereign accounts on bridge hubs. So we don't have any fee calculations on bridge hubs => this issue is not relevant anymore