A solution that I have seen implemented and discussed is a flat, very low fare. High enough to keep people off public transit that are disrespectful of it, but low enough to allow almost anyone to take it thus increasing ridership. An added bonus is when using a transit card to tap on and off, the statistics of ridership are still readily available for governments to better plan infrastructure.
If you pick a low enough price you even decrease the number of fare dodgers, which means that enforcing is not as important or costly.
Here in Queensland we just paid $400 million for a ticketing system that is now used entirely for 50c tickets. The marketing line is that the fee is necessary for analytics, but the cynic in me says that it's probably a combination of secret contract negotiations with a pay-per-tap component, and sunk cost fallacy.
If you pick a low enough price you even decrease the number of fare dodgers, which means that enforcing is not as important or costly.