It rather depends on what you actually want to use it for.  I've got Amazon devices dotted (no pun intended) about the place and I use them, amongst other things, to control my lighting.  That means either buying "special" bulbs/lamps or wall switches, and may also incur the cost of some sort of hub, depending on the devices you prefer. 

If, on the other hand, you just want to be able to ask Alexa questions like "Alexa, what's the weather?" or "Alexa, what's the news?" then all of the apps you'll need (Amazon call them "skills") are free.  Once you've got the device hooked up you just download the skills with your/phone/tablet/laptop/whatever and you're off.

It's a decent time to buy the device(s) at the moment.  The standard Amazon Dot is selling for £24.99 instead of £49.99 and the Echo Show 8 is £59.99 if you want visuals.  I've got a Show 8 on my bedside table to control the lights, act as my alarm clock, and to show me the news etc. without getting out of bed, lazy sod that I am.    I've got Prime, so it will also play Prime Video on its neat little HD screen.