Recs.
Updated
The $50 version of this phone is an Amazon Prime exclusive, and it comes with ads on the lock screen. A similar model is available with no ads for $100.
There is also a higher end version available for $60 with ads, or $110 without ads.
Specs
Pros
Pro Decent hardware subsidized by ads
This is actually a $100 phone which is subsidized by ads on the lock screen. That means that you're getting better hardware at a cheaper price, and all you have to do is put up with ads on your lockscreen. By default there is also an ad widget on the homescren, however this is easily removed.
All ads are provided by Amazon themselves, and not Blu. Each time you press the power button, you'll get a new ad so you aren't even seeing the same ads over and over.
Pro Ads are removable
Currently, ads can be removed if you install an alternative lock screen (downloading one from the Google Play Store works). This may be removed in a future update, but for now this 'hack' is working.