I see you say you want to get from Munich to Frankfurt in order to get cheaper flights out of Germany. So. I would assume you are going to be staying in Munich and will be able to commit to a specific train to Frankfurt. That is exactly what you need to do to take advantage of the lowest "Savings Fare" tickets (as suggested by Joe) on German Rail.
Those Savings Fare tickets are specific to just one train (date and time). The tickets are fully or partially non-refundable, non-exchangeable, so if you miss that specific train, your ticket are worthless. So for people arriving by trans-Atlantic flights, due to the possibility of a delayed flight arrival, I do not recommend Savings Fare tickets. However, since you will already be in Munich and know you will be able to make that train, it's a good deal.
As for Rail & Fly, it's my understanding that the charge a flat fee of about 40€ for the rail segment, and it is an open ticket, valid for the planned day of arrival and the following day, on any train of the Bahn, including an ICE. It's not as inexpensive as a Savings Fare ticket, but has a lot less risk (almost none) and more flexibility.