If you signed an agreement stating that you would repay their costs, you must repay them: you have no grounds to argue about the extent of the cost unless the contract itself gave you grounds, such as by stating that you would have to repay the "reasonable" cost (so then you could challenge whether it was reasonable) or if it put some cap or limit on what you might be charged. But if you signed a contract simply saying that you repay whatever the costs are, you are bound to that agreement and must repay their costs, with the exception that you do not have to pay interest or other late payment fee/penalty which is owed only due to the employer's failure to pay on time.