might it be that you both are not talking about the same?

If I understand "top-up" system correctly you are talking about trying putting money on your prepaid balance to pay the next 4 weeks for your existing rate e.g. via www.telekom.de/aufladen,  but the system only offers 30 EUR as an option correct?

If yes @fdi  actually did not understand it correctly, cause I guess he thinks you need more data volume for your existing rate (e.g. + 5GB up/down). this would be the "speedon" part which is - indeed - more expensive in comparison to the included data volume for your existing rate. 

but why telekom.de/aufladen only offers 15/30/50 EUR and no option (at least I did not find one at the first sight) I also do not know. Easiest way: buy a Telekom (! no other vendor) prepaid code somewhere in the internet or at a gas station or supermarket or whatever, they often offer the possibility to chose the amount or give you 20 EUR gift cards (e.g Cyberport.de offers 20EUR Telekom prepaid cards).