To note with these gas sensors: we'd want a fan to get good ambient air flow into the enclosure containing the sensors!

PM 2.5

Nova PM Sensor SDS011 (0.3 to 10μm) https://components101.com/sensors/nova-pm-sensor-sds011-pinout-features-working-applications-datasheet  only rated 70% humidity | PRICE: approx £18 // some accuracy comparisons https://influencair.be/accuracy-of-the-sds011/

C02

First we want to know what level of C02 should we expect normally for baseline? Well sea level at 15 C and 101325 Pa [verify source!] we have 0.04% C02 in the air.

The EM500-CO2 CO2/Temp/Humidity/Pressure Sensor is popular for forest fire applications, however it costs around 250 euros! So that's no good!

These are all kinda expensive. We've got maybe the MG811 https://sandboxelectronics.com/?p=147 "gives an output in the range of 100-600mV (400—10000ppm CO2)" 400ppm being the baseline CO2 in air. Or perhaps MH-Z16 which is similar. These are in price range of $40-70

TEMPERATURE 

Something like the DHT22 which can also do humidity https://tutorials-raspberrypi.com/raspberry-pi-measure-humidity-temperature-dht11-dht22/ | PRICE $3-4

GROUND MOISTURE 

We've got something like the 'Gravity: Analog Capacitive Soil Moisture Sensor- Corrosion Resistant' sensor for around £6

https://wiki.dfrobot.com/Capacitive_Soil_Moisture_Sensor_SKU_SEN0193

MICROPHONES

I'm thinking of these or similar I2S MEMS Microphone for Raspberry Pi (INMP441) https://makersportal.com/shop/i2s-mems-microphone-for-raspberry-pi-inmp441 for around $8

CAMERA

Well if we do use a raspberry pi then we can use the standard camera 16MP fixed focus for £30 and do a detection of smoke or non-smoke images! Cost is around £30. An alternative is a thermal camera such as MLX90640 Thermal Camera (I2C) https://makersportal.com/shop/mlx90640-thermal-camera-for-raspberry-pi-32-x-24-pixels which sounds quite interesting. But we have a pricing of $90 and very small numbers available for supply.

CONCLUSION

Besides the camera, we seem to be looking at around $80 for sensors. The C02 sensors being most expensive, since these are electrochemical. However, there are some NDIR (nondispersive infrared) sensors that can be used. Although I envisage these to be more costly. The MH-Z16 is actually NDIR and slightly more costly. But we will need to further investigate these C02 sensors.

I'm not really feeling very hopeful to get to under $200 when we add an MCU + board and comms + power!