Devs: whatever it costs to write your code and host it somewhere. Devs do not need cryptocurrency.
Users: free(*) for usernames. Users do not need cryptocurrency. Users must host their app data somewhere in existing cloud storage, but Blockstack PBC runs a free solution for small amounts of data to help them get started.
(*) technically, about $0.005/username in the default onboarding flow, but this cost is covered by Blockstack PBC. This is because by default, username creations are batched and hashed to the underlying blockchain in groups of up to 150 at a time. This costs 1 BTC transaction, but only the tx fee has to be paid. The names themselves are fully replicated to all Blockstack nodes, so as long as the user can reach any node, they can sign in, store data, and look up other users’ data.