If you want to chat auto systems I also have been building one for 18 months now, got it to the stage of 1 year data on device, PH,EC,water temp, air temp, humidity. Can regulate all of those if the right device is added and activated via relays.
I know what you mean about bare parts cost though, mine currently runs over £1200 just in parts, no labour. Most of that is pumps+PH/EC sensors not the box…
I plan on retailing them at some point but the cost will be around the same price point as the currently available dosers although the feature set, accuracy, and utility will be much higher. I hope once I start selling them to be able to get parts cheaper in bulk which should bring down the price some (possibly something like 30%)
If you want a good RH/Temp sensor that you can pull data from easily with something like a Pi, or arduino, you could do worse than a BME280. That link is an ebay listing with a buy it now price of £3 and free delivery…
It is an I2C sensor, humidity and temp to good accuracy, smaller than your fingernail (was designed for mobile phones)
This is a link to the arduino library for the BME280
EDIT : Got the wrong one initially, you want the BME280(which does pressure, temp, and RH%) not the BMP280(which only does pressure, and temp)