Vaultwarden is a Bitwarden-compatible server you run yourself, licensed AGPL-3.0, with no licence fee and nothing sold on top. Its own repository describes it as an “Unofficial Bitwarden compatible server written in Rust”. It is not a Bitwarden product and Bitwarden does not charge for it, which is why it appears on this ledger at $0.00 and why that figure needs a footnote.
The footnote is the server. If it runs on hardware you already own and already leave switched on, the marginal annual cost really is close to nothing. If you rent a virtual server to host it, that rental is your annual bill for a password manager, and you should hold it up against $19.80 before calling this the cheap option.
One piece of fine print worth having: vaultwarden.net is not the project. It is a community-run public instance whose own front page says so, in as many words.