Re: Buying Disney Infinity Game

The pages for Disney Infinity on the Playstation Store each state that you must have the portal/base and at least one figure to play.

"Please note, a platform compatible Disney Infinity Base and a minimum of 1 Disney Infinity Figure is required to play."

(Source: https://store.playstation.com/#!/en-us/games/disney-infinity-30-edition/cid=UP1082-CUSA01705_00-DISNEYINFINITY03)

You don't have to buy a Starter set, however.  Stores like GameStop sell used Infinity portals (roughly $3-5), and used figures (can be as cheap as $2).  As mentioned above, all you need is the portal/base and one or more figures.