Unless things have changed there is as of now no monthly fee for Alexa.
You can speak to Alexa and use Alexa without a monthly fee which is what I do.
You only need to pay a monthly fee for Alexa if you want to listen to music etc but just for the Alexa use you don't need to pay any fees.
However it's not all that expensive for the music and other Alexa things you want.
But you can buy the Alexa and use the Alexa without any monthly fees.
But by getting a monthly subscription to Alexa you'll get more use and benefits out of it.