A Roku is a far more robust streaming device than any based on a game console, a smart TV or a Blu-ray player because it is continuously updated by the manufacturer whereas the others typically are not. There are literally hundreds of official and unofficial channels that can be viewed with a Roku, far more than any of the other devices can offer.
The purchase of a Roku is a one-time purchase. There are no monthly costs other than those you pay to the content providers such as Netflix or Amazon. For some channels such as HBO GO there are no additional costs other than what you pay for the channel on your cable or satellite system.