What am I missing with IDP?

Can you please tell me why so many people are getting charged $400-700 in addition to their $12 a day overseas? What am I missing? In theory the most anyone should pay per billing cycle is $120 where are these extra charges coming from? Data roaming needs to be turned on unless you’re in Wi-Fi so I don’t know what I’m missing? Can someone please help so I don’t have this happen to us.