All of the major US carriers are deemphasizing or doing away with two-year contracts and phone subsidies. This makes the true monthly cost of cellular service more transparent, and it is much higher than prepaid carriers. With the new iPhone a few weeks away and several great Android phones launching recently, it's a great time to try a prepaid plan and save a bunch of money.
For example, AT&T is no longer offering plans with contracts and subsidies on their website. Their plan with unlimited calling and texting and 5GB of data goes for $75 plus taxes and fees. That same exact plan costs $45 on Straight Talk or Cricket, both of which run on AT&T's LTE network. Verizon charges $65 for a plan with 3GB and $80 for 6GB. T-Mobile is a little more reasonable, but their coverage is not as good, especially outside of major metro areas.
These prices no longer include the cost of a phone. If you're comfortable paying the full price of your phone up front (or using your old phone) instead of paying for it monthly, a prepaid plan can clearly save you a lot of money. They don't come with all the extra options of the big carriers (international roaming, shared data, device insurance) but you'll get a lot more for you money. Cricket also offers great family plans at a fraction of the cost of the major carriers.