The only money I see them making on the carts is from the "suggested donation" when they sell the carts to the congregation. A recent article of theirs has stated they've sold over 165k carts, so even if they're only making $10 profit on each one they've made a pretty good chunk of change.
How this balances out with the literature that gets taken from the carts without being paid for, I have no idea. I'd guess they're giving away more literature, but then again everyone talks about how they just sit there and everyone ignores them so maybe not.