Your best bet would probably be Sprint's Simply Unlimited. It's normally $60 for unlimited everything, but for a limited time it's $50 and I believe they grandfather you in under that price for so long as you have a subscription. I'm not 100% positive though, you'll have to find out the terms and conditions.
"The Sprint Simply Unlimited Plan specially drafted for the launch of the new iPhone 6 & iPhone 6 Plus, includes Unlimited Talk, Unlimited Text and Unlimited high-speed Data. This limited time offer plan ends January 15th, 2015."
See more details here:
http://www.whistleout.com/CellPhones/Carriers/Sprint/Personal/Sprint-$50-Simply-Unlimited-iPhone-6
If the plan is really what it says it is, I don't believe you'll find a nation wide carrier any cheaper.
Beyond that, there's prepaid carriers which average around $60 as well for unlimited data.
