I pay $67 a month for 3GB of LTE on T-Mobile (and unlimited 2G after that), on a post-paid plan, for my Android phone. iPhone should be subject to the same plan prices.
Speaking of, that reminded me to downgrade to the 2GB of LTE for free, rather than $10 for 3GB. Thanks GBAtemp. (So now my service will be about $55 a month)