I’m hardly an AT&T partisan, but both they and Verizon charge $10/GB when you go over your data plan limit. Cox wants to charge $0.10/MB, or about $100/GB.
At least, they cut you off before you spend that much, so the most you’ll ever really go over 500MB ($50).
I’m glad Cox is providing a new choice in wireless providers, and their phone plans seem genuinely nice (they buy back your unused minutes every month, up to $20). These data plans are a bit hard to swallow, though.