I buy A LOT of cheap sensors, LED strips and various boards from Aliexpress. So I’m not in for very much. The most I spend is the $5 a month on Nabu Casa.
To give an example, a few months ago I made the electrical heater in my daughters room “smart” and integrated it as a thermostat in HA. Not including the price of the heater the price breakdown was as follows:
Wemos D1 MINI - £2.19
Relay module - £2.14
DHT22 module - £2.84
The actual price is a bit less as I have added the full shipping onto the price for each one. In reality I usually order quite a few of each unit and use them elsewhere.
So for £7.17 I have a smart heater and can see the temp and humidity in my daughters room.