Since the game is not currently free, the most reliable way to get it at a minimal cost is through major digital storefronts:
: A limited version of the game is available on Steam that allows you to play the Versus and Survival multiplayer modes for free. However, this version does not include the full single-player campaign.
While there is currently no official permanent "free" version of the full game, players can often access portions of for free or obtain it through specific digital ownership programs. Official Ways to Play for Free