As a yearly subscriber to Pro Tools studio, the Dolby Atmos Renderer (the piece of software required to mix) cost me $100. I did not have to be vetted to purchase it - Apple & Dolby definitely want people to download this and start mixing in spatial. If you don't have a Pro Tools subscription, I think it costs $300. This software is capable of generating ADM BWF files (this is like the Atmos master format, a very large WAV file with all the beds and objects contained as separate tracks - you can't play them back properly without the Renderer. These are what get submitted to the streaming services via AvidPlay distribution.) and MP4 files with Dolby Digital+/JOC (streaming-quality) audio.
There's another piece of software called the Dolby Media Encoder that will convert ADM to Dolby TrueHD (Blu-Ray quality). You do need to be approved to download this, probably because they only want authoring houses to have it. After a one-month trial, it costs $400 for a yearly-license. That may seem expensive for software, but the DTS:X encoder suite costs $2500.