In Berlin, a city whose public transit uses an honesty system (backed up with unannounced in-vehicle inspections) and fairly low fines for fare dodging, it’s estimated that 3 to 5 percent of journeys aren’t paid for. 

Meanwhile:

London, by contrast, has a closed subway system with ubiquitous barriers and high fines for being caught without a ticket. Unambiguous figures are impossible to come by… but a survey from a decade ago suggested a similar rate of fare evasion in the U.K. capital as Berlin, with 6 percent of riders admitting having dodged a fare.

These are both from an article in Citylab, which also notes that a Dutch city is seeing success by taking the Berlin route – forcing fare dodgers to buy a ticket.

If ~6% of customers fare dodging is the price of doing business, wouldn’t we much rather public transport without all the nasty fences and gates and ticket inspectors?

At the very least we could put that money towards comfier seats.