How to replace a lost iPhone 4s (cheaply)?

Okay, I was keeping my friend's phone for him, but someone stole it T. T I can't find out who did, so I'm looking to replace it obviously. I only have like $300, but from what I understand they cost more than that(?). I obviously can't get a contract because it's not my phone… So what can I do? I asked my friend how much it would be to replace it with his plan, but I haven't gotten a reply yet… I think he has AT&T.