It really depends on the carrier as most phone carriers have no contract devices, I was with Virgin Mobile for a few years and although their phones are pretty good and inexpensive the service was horrible, so you gotta look at what their no contract plans have to offer. When I say service I meant the data speeds. I went and bought a prepaid T-mobile phone and carried over my number from VM, although it took a few days, tested the network with a low end android and I loved it, so I decided to buy the Nexus 4, right now I'm paying $30 a month for 100 minutes and Unlimited Txts/Data but as said it all depends on what carrier and the preference of the plan\
Also the reason why I went with T-mobile is because you can just buy a new phone without having to boot out of your contract and just swap out the SIM cards