Clause of 10% escalation charges in a contarct

I bought a 10 marla house on a 36 months installment plan from a local society, at the time of signing of contract i did ask that what was this 10% as escalation charges? I was told that this will a minimal charge incase of the material going expensive, so i signed the contract ,but now they are asking that i pay 10 5 each year on the value of the hone and not on the material cost? This amount exceeds (even after so many meetings with to around 22 lakh. Is this a normal way of charging the escalation charges ?

The society is defiant in their arguments ,sop far the meetings with them have been only on known persons recomendation basis but no one can beat the society arguments and this is also very strange that so far who ever i know in pakistan have not been able to tell me anything on such escalation charges ?

Can someone help?

Thanks
Best regards