There is also something like https://www.josh.ai/ however its not cheap either. I don't think anything that works well will be cheap.
There is a lot to voice assistants and a number of people have touched on it above. Understanding accents, different languages, logic, doing more than just simple commands (which the wife uses) such as asking for weather in various areas, unit conversion and other random questions.
There are also hardware considerations. These small boxes are more than just a voice command processor and a mic. They have processors and multiple mics to help them recognize who is talking, or help distinguish between a tv and a person or even where the voice is coming from to help with multi room installations.
On paper it sounds simple, but even Amazon and Google struggle to get it perfect.