Recently it was "free" on GOG for Amazon Prime subscribers:
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NOTE: There's 2 "complete the set" bundles, for owners of Slay the Spire or Inkbound, which make this even cheaper. I already had StS and I got the Slay the Spire x Monster Train bundle, which was about 10% lower than Monster Train would've been.
https://store.steampowered.com/app/1102190/Monster_Train/
Slay the Spire x Monster Train bundle
Shiny Shoe Originals Bundle (Inkbound + Monster Train)
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