Further to my piece on smell sensing tech, it seems that breathalyzers, which use gas sensors like this one: Alcohol Gas Sensor, are getting smaller. This one attaches to a smartphone, fits in a pocket and costs $35. (Via Interesting Engineering)
That’s not the cheapest one out there — this BACtrack Ultra-Portable Personal Keychain Breathalyzer probably is — but I think it’s probably the cheapest that connects to a phone.
Some more links on the matter: