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I'm starting to explore IoT in my home, the first step being to connect a raspberry pi 3 B+ to a DHT22 temp/humidity, record DHT readings in a database which I then surf via a node server running on the pi. All good.

I got help from e.g.: http://www.circuitbasics.com/how-to-set-up-the-dht11-humidity-sensor-on-the-raspberry-pi/

I'd like to broaden this now to add some cheap wireless temperature/humidity sensors, but most of the ones I've found appear to be locked to their manufacturer, e.g. Xiomi.

The answer to this question suggests connecting a RF module to the pi to receive data from sensors. This instructable also seems promising, suggesting purchasing these transmitter/receivers.

So maybe I'd be able to connect something like this sensor to such a setup?

Will using RF 433MHz be a good way for me to keep costs down?

My solution to this was to use ESP8266 boards with various temp sensors like a TMP102 or LM75A. The ESP chips then send their data using MQTT to a Pi which is running Home Assistant.

Here are some links that might help:

https://www.hackster.io/colinodell/mqtt-temperature-and-humidity-monitor-for-home-assistant-27b8d1

https://www.home-assistant.io/blog/2015/10/11/measure-temperature-with-esp8266-and-report-to-mqtt/

As always with a wireless solution, the critical parameters for selecting a solution are required range and power consumption.

You might be able to identify a cheaper pair of sensors, or even find a MCU which is integrated with the sensors you need (and a radio) if you're lucky.

The sensors you found seem to simply broadcast digital data continuously, so whilst you could connect one of these to a 433 MHz transmitter, you will only get one channel.

A more standard approach would be to use a Wifi/BLE/ZigBee/ah-hoc wireless module which exposes a limited amount of spare processing power, and use the wireless module to control when to send data samples. These modules come in at $2-3 (wifi seems cheaper than BLE) and ought to be suitable for battery power applications. You will of course need to learn a new software/toolchain approach, but with luck your endpoint application is a simple example which you can just import with minimal modification.

Once you accept an MCU into your endpoint, you can use any sensor, using a 'single pin self timed', SPI/I2C digital interface, or even an analogue input if you find a device with a spare ADC input. In fact, there is a wide range of options which will allow you to try and compete with this pre-built ESP8266 implementation, but you will struggle to come in at better than the sort of prices that Xiomi are achieving. I guess that the device I linked there is 'open' to some extent, but need to wait and see.