Google is currently running a promotion on the Google Store that could net you $50 in Google Play credit when you purchase specific devices.
Taking advantage of the offer is pretty simple: you simply purchase one of the above devices and then Google will email you a promo code that you can then apply to your Google Play account.
It is important to note the fine print on this offer, however. The deal runs through March 31, and you'll have to add the Google Play credit to your wallet by April 20 use it by September 1 or risk losing it. Additionally, the credit only applies to Google Play, and not the recently-launched Google Store through which you'll be purchasing your device. In any event, if this sounds like a good deal to you, feel free to hit up the source link below to get cracking.
Source: Google Store