The vast majority of ISPs no longer have data caps, so their customers are free to use as much data as they like without paying any additional charges. If you're one of the
tiny percentage of broadband users who're still stuck with a data cap, this information will be relevant to you:
https://goodhomeautomation.com/how-much-bandwidth-does-alexa-use/
This article, from The Grauniad, covers the privacy concerns that some people have associated with Alexa devices (and their rivals):
https://www.theguardian.com/technology/2019/oct/09/alexa-are-you-invading-my-privacy-the-dark-side-of-our-voice-assistants
I've personally got nothing against them but I simply don't think that I'd gain much from having one. I've got an internet radio which, along with YouTube, serves my music needs well enough and, as I count myself as being rather good at using Google, I don't need Alexa to answer questions for me.
Further, as I live in a one bedroom 'starter home', I'm never very far away from my computer or my internet radio anyway. (I've also got no need of devices that work with Alexa, such as those that can dim the lights in a room upon a voice command. I'm perfectly capably of standing up, walking a few feet and using the dimmer switch on the wall!).
I'm sure that there will be people answering your question here for whom Alexa-enabled devices are either a really useful tool or, at least, an entertaining novelty. I'm not arguing against them at all; I'm simply saying that I can't see that I'd benefit a great deal from having one.