Intersting one here. Two perspectives from my side (and I have a degree of experience on speeding fimes in Germany..!!!):

1. According to the official table of fines (yes, we do have that kind of thing over here) the amount due for exceeding the limit by up to 30kmh is €80. However, since this was in a hire car, it is quite possible that the hire car company has levied some kind of an admin charge on you. To whom is it that you are being asked to pay money?

2. As has been correctly pointed out if a) you plan to return to Germany in the forseeable future and b) your name is linked to the fine then if you chose not to pay, you stand a very good chance of being detained by Border Police when you next arrive and having to pay even more (we are quite efficient at that kind of thing over here).

So - and I believe this is the way you are leaning anyway - "fair cop and just pay up"