The price of the iPhone 5C is $575. With your current $30 plan, you're eligible for either a Tab S or a Tab M.
With a Tab S, you'd be required to pay $425 for your phone upfront and your monthly bill stays the same.
With Tab M, you'd be required to pay $275 upfront and your monthly bill would rise to $35 for 24 months (or if you clear your tab balance early) due to a $5/month Tab M fee. After the 24 months, your monthly cost returns to $30.