Price.com earns a commission when a shopper buys from a participating store after visiting through Price.com. The store pays that commission for the referral, and Price.com shares a portion of it back with you as Cash Back.
That is why Cash Back costs you nothing. It is also why a purchase only tracks if you start your shopping trip at Price.com or have the browser extension turned on.
Price.com Pro memberships are a second source of revenue. Pro is optional, and you never need it to earn Cash Back.