There are apps that measure data use and can be set to alert you.
The real problem is app writers who assume everyone has unlimited internet and cellular providers who are happy when your bill is high. I am of a certain age so I know it is possible to live for a few hours a day without access to internet. I have an iPhone with no cellular data service so no worries about high, unexpected charges. Except roaming. I have a pay as I go "plan" so everything stops the moment I use up the money in my account. Studies show that folks with plans end up paying about the pay as you go rate because they purchase more plan than they normally use to protect themselves from occasional heavy use.