CryptPad is an open source document collaboration service that can be self-hosted. CryptPad is zero-knowledge, which means that the operator cannot read the content written on the site. Since it can be self-hosted there are many instances. The most known, which is also driven by the main developers however is cryptpad.fr.
The offer of CryptPad is free for guests and registered users. As a registered user you get 1GB of storage for free. CryptPad also offers a paid plan starting at 5€ per month with more storage. Since CryptPad is an open source project, donations are also accepted.
Read more on the pricing page of the service.