phd: a full time job for 50% salary??!

Hello, i’m applying for a PhD position in Würzburg University and they just told me what title says: it is a full time job for 50% contract, salary according to tv-l e13 (didn’t even tell me the number, i had to search it: 1250€ /month after tax!). im stunned!! is that common practice?! how is this even legal.. does anyone have similar experience? how did it work out for u?