Thank you for using JustAnswer.
I'm sorry to hear about your situation. If by "legal reason", you mean to ask if it's legal, yes, it is. They might even have a legal reason (that being that they have a contractual obligation to do so). But more likely it's an economic reason. If you've ever bought a plane ticket you know that they don't charge the same price twice, even if it's the same type of seat on the same flight, etc... Companies are free to "price discriminate" in a manner that maximizes their revenue, so long as they're not discriminating on the basis of race, religion, gender or disability.
The term for this is actually "price discrimination", with "discrimination" merely meaning treating two similarly situated individuals differently. Discrimination is not illegal. Only illegal discrimination (on the basis of race, age, religion, gender, disability) is illegal.
Price discrimination is a commonplace practice that is presumptively lawful, save under special circumstances discussed below. It occurs when a seller of products regularly offers lower prices to its preferred customers and higher prices to the others when selling them the same or similar products at around the same time. SeeContinental Baking Co. v. Old Homestead Bread Co., 476 F.2d 97, 103 (10th Cir., 1973) (“The term ‘price discrimination’ means no more than price differentiation, or the charging of different prices to different customers for goods of like grade and quality.”) (citing FTC v. Anheuser-Busch, Inc., 363 U.S. 536,***** 1267 (1960)).
Furthermore, the law is restrictive in nature, not permissive. That means that you're not going to find a law that says that someone can do something. You'll only find a law that says someone can't or must do something. Laws restrict rights. As there is no law that says that they can't discriminate on price, it IS legal for them to do this. Now if you're under contract with them, that means that you're going to have to continue until the contract is over. Once it is, you can renegotiate or go with another seller.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (good or better). Thank you, ***** ***** luck to you!