I have the same contract with Sprint as you do. What phone are you using? I have an EVO 4G, and there is a $10 charge for "enhanced data" on top of that, though I have a discount and my bill runs about $75 per month.
I agree that AT&T is high compared to Sprint, but Sprint is mum on WP8, so there isn't much choice if you want a 920.
Its an HTC Arrive. Is $69.99 base, $10 for "enhanced data", $5 for "spending limit", and they add their surcharges and taxes on top of that to come out at $98 and change per month.
Net 10 is $45 a month. Unlimited everything. Data is really limited to 2-3GB a month though.
Is it confirmed that the Lumia 920 will work with Net10 sims without unlocking? (I have no idea who Net10. I have only ever seen their crap-droids in Best Buy)