Charged for 4g service, no 4g service available.

When I purchased my HTC Evo in Nov nearly 2 years ago, I was told 4g was coming and that I had to pay an extra $10 a month, even though it wouldn't be in New Mexico until the next May.  I agreed, but I can't even get 3G service in all of New Mexico, let alone 4G anywhere - I want my $10/month returned for the entire contract length of 24 months, or $240, preferably with interest.  I have tried to rectify this through Sprint more than once and have been told that no changes to the contract will be made, despite changes to their plans to implement 4G in New Mexico.