How much does Roku cost monthly?

Could you please clarify the cost of Roku's services on a monthly basis? I understand that Roku offers streaming devices and services, but I'm particularly interested in knowing the monthly expenses associated with their offerings. Is there a flat fee for access to their platform, or do the costs vary depending on the types of channels or content one subscribes to? Additionally, are there any hidden fees or additional charges that I should be aware of? Your insight would be greatly appreciated.