This is similar to what Amazon's been doing for a long time with its Fire tablets, which are available at a lower price if the buyer accepts ads.
The $50 discount is available to Prime members who pre-order any of the following ad-supported versions of the Moto G4 and Blu R1 HD. Pre-ordered phones are supposed to ship July 12.
BLU R1 HD 8 GB $49.99 with Prime and ads ($99.99 without)
BLU R1 HD 16 GB $59.99 with Prime and ads ($109.99 without)
Moto G4 16 GB $149.99 with Prime and ads ($199.99 without)
Moto G4 32 GB $179.99 with Prime and ads ($299.99 without)
In addition, according to Android Police, Amazon is offering an additional $25 off either Moto G4 model if you order today June 29. The extra discount is not shown on the Amazon site until you add the Phone to the shopping cart.
We covered the Moto G4's launch yesterday, but to summarize it features Android Marshmallow, a Snapdragon 617 SoC (System on Chip, 2 GB of RAM, 5.5" 1920 x 1080 Gorilla Glass 3 screen, SD card slot and 13 MP main and 5 MP front facing cameras and support for CDMA and GSM carriers worldwide.
The BLU R1 HD is also a new phone that looks to be a great value. It's a dual-SIM device that runs Marshmallow and has a 1.3 Ghz quad-core MediaTek 6735 SoC, 8 or 16 GB of internal memory, 1 or 2 GB RAM, SD card slot, 5" 720 x 1280 px curved Gorilla Glass screen, and 8 MP and 5 MP cameras. It will support LTE bands 2/4/7/17 at launch. Support for T-Mobile's band 12 is supposed to be enabled by an OTA update "shortly after launch" You should definitely choose the 16 GB model which is only $10 more than the 8 GB variant and comes with 2 GB of RAM instead of 1 GB.
One of the main advantages of buying factor unlocked non-carrier branded phones is that they typically have little bloatware. Is a one time saving of $50 worth seeing ads every time you wake up your phone? You may not have to make that choice however. It's apparently fairly easy to remove the lockscreen ads from at least some Fire tablets. If the same is true with these phones, I'm sure ad-removal instructions will be posted on XDA-Developers and other Android enthusiast websites within days of the Moto G4 and Blu R1 HD launch on July 12.
Sources: Amazon, BLU, Image: BusinessWire