GitHub

Half-Life: Alyx

Half-Life: Alyx has mod support through the Steam Workshop. This mod support isn't as extensive as other Source Games, but it has been said this will be improved in the future.

Of course the real cost in Half-Life: Alyx is the VR setup, mainly the Index, which the game was built for.
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Extensive official mod support.
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You buy it and that's it. No paid DLC. No microtransactions.