That's not strictly true. It doesn't cost them _much_ but holding and tracking other people's money has a cost.
That said, I think the better answer is to send it to the state as unclaimed property.
That said, I think the better answer is to send it to the state as unclaimed property.
When you have as many users as PayPal does, in aggregate those non-zero balances are a mountain of money to play with. It's not a cost, it's opportunity for profit.