To avoid surprises with mobile data costs, you can set your phone to notify you or close the mobile data connection when you have transferred a certain amount of data using your mobile data connection.
Select 
> Settings.
Select Connectivity > Mobile data tracker > Data allowance.
Select Define in megabytes or Define in gigabytes, then type in the limit in the field.
Select When allowance reached, then select Show warning or Switch mobile data off.
The limit is phone-specific. If you use your SIM in another phone, you need to set the limits in that phone.
Your network service provider may count mobile data use differently than your phone does. For info about data costs, contact your network service provider.