Extremely cheap Home Kit compatible system: Xiaomi Aqara
If you want to buy a Smart Home System with many different sensors, you can take a look at the Xiaomi Aqara, which is so cheap that it might put other suppliers under pressure.
The hub is currently available for around 40 dollars on Aliexpress, individual sensors cost around 10 dollars. Thus it lies substantially under the price of established systems. And it is fully Home Kit suitable and certified by Apple! So you can buy more sensors for little money and automate more sensible actions if the budget is limited.
You can buy the Hub without doubt, but be careful to buy the new Home Kit version. There is also an older one, which looks quite similar, but does not support Apples smart home standard. This is the one to get:
Buy on Aliexpress: Xiaomi Aqara Home Kit