When renting a car in Germany, and many other countries, if you want to guarantee a low price you should look forward to stay with the car between Saturday and Sunday. Usually it is the leisure client that stays the night from Saturday to Sunday, so the business client (normally willing to pay more) cannot benefit from this offer.
The maximum for these special prices can be 72 hours, which is enough for three nights, so if you want more than this specific number of hours you may end up paying more 50%.
These are some examples of prices:
Frankfurt Airport - Pick and Return in April (Prices may vary, specially during holidays)
Europcar - Audi A1
3 Days including Weekend - €135 (€75 from Friday to Monday)
3 Weekdays - €194
5 Days (weekend or not) - €306
Curiously in the Alamo website you will find different prices for the same dates, but they are the same in the Frankfurt Airport.
Hertz - Ford Fiesta
3 Days including Weekend - €78
3 Weekdays - €166
5 Days (weekend or not) - €252
Budget - Ford Fiesta
3 Days including Weekend - €69
3 Weekdays - €137
5 Days (weekend or not) - €237
As you can see prices can be very different, but don't take these prices so seriously, the conditions may vary from company to company, and you can see the promotions in the website. Other factors like the currency you are using and the country where you are from may change your price.
Considering the biggest car rental companies and the ones that are present at the Frankfurt Airport (biggest in Germany), this is the List of Car Rental Companies easily reached:
Europcar (Alamo / National)
or you can just visit a aggregate website like RentalCars.com
My experience has been good with Europcar, for both times I went to Germany and rented the car, Europcar was doing promotions in Germany(25% discount).
You can also do some homework for them and do a "online check in", so when you get there, they have almost all the information they need.