I would do as toedtoes suggested. If that fails, you either pay the extra $80, or ask for a full refund.
Because you are so far away, going to court in NH for breach of contract is totally impractical. No, it is not right. They would be in breach of contract (unless they had something in writing at the time you paid which said they had the right to raise rates without notice on prepaid reservations), but it isn't worthwhile for you to pursue it.
Personally, I guess I would ask for the refund first if I had anywhere else I could stay instead, simply because I would not want to give my business to such underhanded people.