The EV3 Expansion Set is expensive to buy new and has been retired, so if you are just getting in to it you could go to Ebay or BrickLink for a cheaper set that's used.

BrickLink: Mindstorm EV3 

Ebay: Mindstorm EV3 

Facebook Marketplace: LEGO Mindstorms - Education EV3 Core Set (item 45544)

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