15-20€/month gets you 1-3GB of traffic from a cheaper operator like 1&1. "Premium" operators like Vodafone start at around 30€/month for 1GB. Whether you find that reasonable depends on what you compare it to (I'm used to 10€/month without traffic limits), but either way those traffic limits are nowhere near enough to replace free Wi-Fi for work purposes.
I pay 39 EUR/month for 40GB of data with free roaming in Europe and the US. Unused data carries over up to 80GB on top of the 40 included. So you can travel around with about 100GB of data which is significantly more than you get in Germany for that price.
