Obligation to pay more under contract than realised
I signed a contract thinking I had to pay one amount of money, but it turns out I actually have to pay much more. Do I have to pay the higher amount if it was an honest mistake?
It is impossible to give you any meaningful advice without more information, but in general terms, it depends on what led to you to believe that you had to pay the lower amount. If it was your own carelessness in failing to read the contract properly, then most likely you will have to pay.
If the other party said or did anything to make you believe that you had to pay less than what was actually provided in the contract, or if they knew that you were under the wrong impression but did nothing to correct your understanding, then you may have a remedy in law. In that case, you should go and see a solicitor and talk through the situation.