If you are in the US, you might want to run to your nearest Microsoft Store (or go to MicrosoftStore.com) where you can currently buy a contract-free Nokia Lumia 520 for $59, including a free case.
This phone is on a pay-as-you-go deal, and you might choose not to go - which means you can still use it on WiFi. Best of all, it can be upgraded to Windows Phone 8.1, so it's ideal for testing your new Windows Phone apps on.
There is a model for T-Mobile too, but it's $69. I mean, that's an outrageous price for a Windows Phone that can run the very latest OS. And by "outrageous" I mean "also ridiculously cheap".
I've used the 520, and it's a solid little phone. Sure, it lacks a front-facing camera (which is why when I Skype my mother who has one, I just get images of her surprised pet cat getting yelled at), but come on - 60 bucks!