Advise, please, on the cheapest way to travel from Frankfurt to Vienna. Not driving.
I also believe you can get a night train that is reasonable which also saves you the cost of a hotel for a night. I'm planning on the night train from Vienna to Frankfurt next year but they won't let you book but about 90 days in advance.
If you can book far enough ahead and are flexible as to date and time, the Bahn has Europa-Spezial fares as low as €39 2nd class and €59 1st class. But you have to book early; tickets are limited at this fare. I checked one date and only found €69 2nd class remaining, but they did have €59 1st class. Reservations might be a few € on top of that, but that is otherwise the complete charge - no taxes and fees, no booking charge, no checked luggage charge, carry on all you want.
train
What day of the week, how many people?