For CO2, which I've researched the most out of the various things that interest me, I'd want a Senseair K30 or S8 Commercial, an Amphenol Telaire T6613 or T6713, or a CozIR-A or CozIR-LP sensor.

For low cost, I'd go with the Senseair S8 Residential. It's not fantastically accurate but it's still a "real" sensor and you can get it for $20 from China.

For other aspects of air quality, I haven't finished researching the options yet.