It seems like every day there is a new deal from one of the U.S mobile carriers. Today's deal comes from Sprint, and it will save families a ton of money, but there's a catch: You can't be a current Sprint customer.

Starting July 31, a family of four can get four lines of unlimited talk, text, and 10GB of data for $100 per month. If 10GB isn't enough, you can add 30GB of additional data (40GB total) for an extra $20 per month. The data is shared, meaning each family member is free to consume data based on their individual usage up to the monthly limit for the entire family.

To be eligible for this deal, you will need to port all four numbers to Sprint from another carrier. To sweeten the pot, Sprint will pay off your old phone and wireless contract so you can make the switch. That form of payment will come in the form of an American Express Rewards card. Customers can choose new phones with Sprint Lease or Sprint Easy Pay, and the $15-per-month, per-line access charges will be waived as long as the you're enrolled in the plan.

Sprint claims customers will save over $480 per year over Verizon Wireless and $720 per year over AT&T when using a similar four-line, 10GB monthly plan. T-Mobile would cost the same with unlimited data, but the speed is throttled after 1GB. Sprint won't throttle data at all.

This deal is for a limited time, but Sprint didn't specify how long. If you don't mind switching carriers, this one might be a good deal. That is, of course, if Sprint has decent service in your area.