Some relays are paid so that you pay 1 sat for every sent message. But other relays are free and toss messages without cost. I was initially critical of the pay-per-message scheme, but after using it I see its value and believe it can make conversations higher quality. It also helps fund the operation of the relays which ensures they'll be there when you need them, and operating optimally. It makes perfect sense in the v4v paradigm. But again, you can opt to use free relays, like Damus and 0xChat, if you prefer.