Voxeo undercharging Twilio 50%, what's their profit margin on SMS?

Late on Friday night I browsed over to smsified.com since I'd been keeping an eye on its minimal "1 cent per SMS coming soon" webpage. That night I was surprised to see the service had quietly launched and Tropo's pricing had also already dropped since Voxeo owns both of them. As of Monday, Twilio hasn't followed their lead (which I've been waiting for since my app uses Twilio). I've read about SMS costing the carriers almost nothing, but I'm wondering what the actual profit margins are. How low can they go?