This not true for all carriers. T-mobile has cheaper non-contract plans.
No-contract is 50 dollars for unlimited everything ($70 if you want 4G): http://prepaid-phones.t-mobile.com/monthly-4g-plans
With a contract it is 90 dollars for unlimited everything (which I believe is roughly standard across the industry): http://www.t-mobile.com/shop/plans/individual-plans.aspx