I have prepaid, so I pay exactly nothing if I don't use it. I usually buy 50MB (yes, that's MB) for one month for 1,99€ but most of the time 50MB is too much for me. My last three months were 25MB, 45MB and 14MB.
@musicmatze I could use that within 5 seconds. I've a 15 GB on one Sim (Car) and an unlimited on the other Sim and usage was 317 GB in the last month. Usually backups, data syncing and audio/video calls, a few moments Netflix stream sometimes.