Modern smartphones today have a full price of $129 (Moto E) up to $850 (64GB Apple iPhone 5S). If you want to spend just $100, then the Nokia Lumia 635 should be near the top of your list as it offers much more than $100 worth of value.
ZDNet's Jo Best posted her thoughts on the 635 for the UK market, and the AT&T GoPhone model offers the same capabilities and functionality. The US carrier market is still primarily focused on the subsidized phone model, but you are being fooled if you think new smartphones from Apple, Samsung, HTC, LG, and others only cost you $199. T-Mobile started the trend and now others are starting to offer full price, no-contract smartphones so that consumers see the real price of devices.
$100 for a smartphone with no contract required is a fantastic deal. The Nokia Lumia 635 is locked to AT&T, but you are in complete control of your service plan.
With Cortana, HERE Maps, and free Skydrive storage it is pretty easy to quickly justify the $100 price. Not to mention that you get a fully functional smartphone with access to hundreds of thousands of apps, cool customizability, and easy multiple account synchronization. The Lumia 635 makes a great backup phone for those who also might purchase a high end expensive smartphone.
Like Jo mentioned, the camera isn't great, the display is just OK, and there are still some app gaps. Windows Phone is very responsive, you can expand the storage memory up to 128GB with a microSD card, and the loaded Windows Phone 8.1 operating system is fantastic.
More ZDNet Nokia Lumia coverage