I tried setting the throughput for a Cassandra table in Azure Cosmos DB settings to below 400 but it does not seem to work. The save button becomes inactive, disallowing this.
I'm running a test server that will never generate such a high throughput. Is it possible to set the minimal throughput to less than 400 RU/s?
No, you can't. 400 is the absolute minimum. Even when directly creating the database through the REST API you'll get an error if you set it any lower than 400.
You could however opt to try the serverless preview where you'll only pay for the RU's used. If you purely use it for development where every now and then there's a few calls it'll very likely be cheaper, although always calculate it before so you won't run into a nasty suprise. You can opt for it when creating a new cosmos resource. I don't think it's possible to change it in an existing resource.