Depends on the channel. Netflix is around $12 per month. Prime video is part of Amazon Prime, which has either a monthly or yearly cost, but includes far more than streaming. Hulu has a monthly fee that varies with the selected package.
There's no subscription to use the Roku itself. And there are many channels that offer media for free, with ads. But every channel with a subscription has their own fees, so there's no set price that anyone can tell you.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.