Even after cutting the medallion fee in half, NYC would have $9000/cab to devote to policing taxis. Assuming the 4x yearly safety inspection costs $1000/inspection (at a rate of, say, $333/hour for the mechanic), that leaves $5000/year/taxi for police work.

So for every 15 cabs, the city could spend $75,000 on a dedicated undercover taxi inspector. Lets say this undercover inspector works 250 days/year, and rides 3x/day. Thus, every taxi is subjected to an undercover inspection every single day (on average, assuing 250 working days/year). Assuming an average fare of $12 and taking revenues of $90,000/year (see my previous link), a taxi driver has 30 fares/day, one of which is an undercover inspector.

Each time he uses shady tactics, his odds of being caught are 1/30. 50% of cheating cabbies will be caught before they manage to cheat even 20 customers.

Also, if you don't feel that $35k/year (higher household earnings than 37% of the US) is a living wage, then what is?