The cost of a new phone should not keep any person tied to a particular wireless carrier. Today flagship devices cost between $350 and $1,000.
After the announcement of the Nexus 6 and iPad Air 2 it is technologically possible for one mobile devices to work on all the major US wireless carriers.
Starting in 2016 all mobile devices that can connect to a wireless carrier, should be able to connect to at least the four major carriers. The four major carriers at the time of this writing are; Verizon, AT&T, Sprint, and T-Mobile.