Germany has about 1000 railway locomotives operating 40k runs a day. A conductor job pays on average 50k€/yr. Most trains I've seen have more than one conductor, and you need to plan for weekends, shifts, time off, etc. but the lower bound should be easy to agree on 50M€/yr or 12.5M€ for the duration of the program? Then there is the entire infra to sell tickets online, in machines and in person.

At the same time there are about 2 million rail customers a day, so the 9€ ticket will bring 18M€. I'd say they probably break even.

Math is fun, but the entire point is a bit moot. Germany is a civilised country and if you laid off the entire staff for three summer months with no pay, the union would eat you for breakfast.

As a rule of thumb: anytime you see a human doing a manual task, it's expensive.

I'd also be curious for actual numbers, but certainly on the commuter GO Train in Ontario, it's basically a 3-4 person staffing arrangement, with one person at the back, one in the middle in the accessible coach, and 1-2 in the locomotive. Adding someone whose job it is to roam up and down the train checking tickets adds 25% to the personnel cost, which is why most fares aren't checked.

But quite apart from the person and their ticket-scanner, there's also the whole infrastructure associated with fare taking— fare-free advocates argue that if you get rid of the machines and websites and apps, and all the associated upkeep, it ends up being a wash. Of course this only counts if you're getting rid of all fares rather than doing a one-off summer special.

You don’t actually check any tickets, you just pretend that tickets will be checked at random, regularly. Put up signs to with heavy fines & travel bans for breaking the rules, run some fake newspaper stories of heavy enforcement etc. Make it socially embarrassing not to pay etc etc

This ticket only applies to regional trains. Regional trains usually have a staff of 1 (sitting in the locomotive, driving the train). Tickets are checked by random patrols, which don’t actually cost that much (in each given train at a given time you’ll have a chance of encountering them every few weeks at most).

Often, not even that much, as the check is just one of many jobs the personal has to execute. The other jobs won't disappear just because they might not check now.