What cheap phone should I get for flashing?

Okay, I have a Nexus 4 right now but I wanna start flashing and installing custom ROMs. Since there is still no good data backup, I want to use a cheap phone for trying stuff. Which one should I get to be able to install miui? I kinda like the interface, even if it looks a lot like iOS 7.

I thought about the Nexus S or One but they still cost over $100. I would like to pay... maybe $50. As I said, I don't need great features or KitKat, just a phone which supports miui. Any ideas?

Thanks guys.