It doesn't matter what anybody else pays. They have different minutes. Different text messaging needs. Different features. Different amount of people on their plans. They will pay different amounts based on all those factors.
For you, your current bill will increase $30 if you go with 2GB of data, $50 if you choose 5GB of data, or $80 if you decide you need 10GB of data. Most people go with 2GB. Your data allowance doubles for a 4G phone, but the cost is the same.