That would be much cheaper than a union bus driver who can clear 100k.
Safer too since bus drivers often go tons of overtime which isn't great for being alert.
https://news.ycombinator.com/item?id=30856522 (citations)
That's typically enough to stop most weird behavior like throwing trash on the ground.
This isn't exactly a direct source, but it's listing the hourly pay at 25.26$.
You wouldn't even need them to ride every single bus, just enough to deter weird behavior.
Even if hypothetically they cost as much as a real bus driver, this would be much safer. A bus driver doing overtime and possibly texting people isn't too hard to beat.