itch.io

i will just post this from steam forum

Configuring Additional DLC Depots After following the depot documentation you should have one DLC Depot added to your Base App. If you need to add additional DLC depots to support alternate languages, OS's, or architectures, follow the steps below:

1. Open the Steamworks App Admin for the base App (All depots are managed in the base App, not in the DLC).
2. Navigate into Steamworks Settings, SteamPipe, Depots.
3. Click the Add New Depot button, set the name, and click OK. Give it a name similar to the existing DLC depot so you can easily identify it.
4. Set the correct language, OS or architecture.
5. Choose the correct DLC from the dropdown box to associate this depot with the DLC.
6. Click Save Changes. Note that the Steam client will not know about the new depot until you have published your changes.
7. Add the depots to any packages which need to contain your depot (e.g. Developer Comp package)
8. On your build machine, add a new depot build script for the new DLC depot.
9. Add that new depot build script to your base game app build script.
10. Complete any remaining tasks to upload the new Depot files, set the build live, etc.

Note: All DLC depots reside within the Base App Depot list. There is no way to have Depots reside within your DLC App directly.

Source: https://partner.steamgames.com/doc/store/application/dlc
Referenced "Depot Documentation": https://partner.steamgames.com/doc/store/application/depots

So @Topic: No it does not cost you extra $100 to release a DLC. You simply update the file-pool of your base game and create a new depot for a DLC that should be buyable. Then you link your desired DLC files with said depot and there you go