We started testing electrochemical NO and NO2 sensors from Alpha Sense in our lab and are in the early stages to design a unit that would also measure these gases but we are many months away to have something in the market.
The EPA limit for NO2 is 100ppb; have you found a sensor that can reliably report levels near that threshhold for less than $500? I haven't. Also the ones that don't require frequent recalibration seemed to be even more expensive.