Mobile data became more important then how many minutes you can talk or messages. Everything we do, we need mobile data. The problem is, when you finish the small amount of data that you have on your contract, if you still use it, you will have a huge bill. Any additional data is very expensive ! Galaxy S3, S4 and S5 make it easier to you, as you can set a limit of mobile data you can consume.
If you are a S3 user, enter Settings and tap Connections tab. Then choose Data Usage and then Set Mobile Data Limit. There you can enter any data limit you like before you get a warning.
If you are a S4 user, there are the same steps. It can be a little bit different depending on what Android version you are or if you have a custom ROM.
For the new Galaxy S5, as it has TouchWiz, the process is different. Hit Settings, choose Quick Settings->Data Usage and Set Mobile Data Limit. You could see this ultimate setting only when you will have a SIM card inserted in the device.