Xiaomi gateway + sensor is so far one of the cheapest non-DIY, thanks to @Ondrej_Pecta and @pboos who are working on bindings for Xiaomi 
The cheapest gateway I found on Aliexpress is around 28 usd.
The battery powered temperature + humidity sensors are $10 each.
You can add I think up to 30 devices to the gateway (but this needs to be verified…) like motion sensors, door contacts and wireless switches.
Although, you need a clunky smartphone app (poorly translated to English) to get things connected.
But I thought it may be worth trying anyway