Nexus 5 can run you $50 on Sprint, but only if you switch
Sprint lists the Nexus 5 with a price tag as low as $49.99 for the 16 GB version on a two-year contract with its unlimited data offerings. To arrive at that price, though, Sprint takes the $449.99 price of the Nexus 5 (yep, no subsidies from Google on carrier versions, it seems), and applies an instant saving plus $50 mail-in rebate combo.
The instant savings amount to $350, of which, however, a Benjamin is applied if you bring your number from another carrier, with the current Switch&Save promotion. Thus the actual price with a two-year contract extension is $150 for the handset, which doesn't differ all that much from the other flagships on Sprint, such as the Galaxy S4, which will run you $200, or $100 with the Switch&Save in its turn.
Overall, this confirms that the best place to get a Nexus 5 is from the Play Store, as Google is subsidizing the handset, or perhaps selling it at cost, to the tune of at least $50 (or euros and pounds on the Old Continent), making its own offering rather sticky and irresistible.