So some costs - a decent Echo can sometimes be on sale for about £60. On top of that FireTV £25, controllable plug sockets, about £15 each, I already have a Hive heating system otherwise you are looking at £100-£150. Lightbulbs depends on the system but ~£15 each. Fart noises, free!
You don't have to jump in all the way with home automation. You can start off with just an Echo and build up if you think you need it. We don't have light bulbs because I want to also be able to turn a light on with a switch, we do use the plus to control a table lamp though. I also have a home security camera, but that is not Alexa compatible, but it does not really need to be.