I feel like a lot of the time it's more expensive to hire fare inspectors than the amount of potential recovered losses. However, most places still hire fare inspectors because the cruelty is the point.
Like, if a fare inspector is $100,000 per year, and fare is $3, that's a lot of fares to catch before it's worth it.
Saying that, I used to not pay a lot. Now I do, mostly because I can afford it. I didn't used to be able to.