Maybe you are not aware of the 9 euro ticket, so this is the hack: at the beginning of June and the end of August, it will be possible to buy the special passes, valid for one calendar month each, for just 9 euros (i.e. 27 euros for the three months).
The ticket will be valid on all forms of public transport across Germany, including buses, U-Bahns, S-Bahns, trams, and local and regional trains. With the 9-euro ticket, you can use these forms of transport nationwide, but long-distance transportation services, such as ICE, IC and EC trains run by Deutsche Bahn, and FlixTrains and FlixBuses, are not covered. ( Remarks: the ticket will always be issued for the respective calendar month. So, for instance, if you buy a ticket in mid-June, it will only be valid until the end of June, and you will be required to purchase a new ticket for July. )
This Ticket will be will be sold online as well as on tradition sales Channels.