498a filed by father in-law

No you will not have to pay any thing for these 498a allegations, unless they are able to make you agree to their terms.

For 498A , state will appoint a lawyer for wife (Public prosectory) , so to fight 498A case , for wife there is no expenditure. Though her counsel may assist PP and charge some thing from her but again its can not be demanded from husband.

For 498A husband, yes he will have to hire a lawyer to defend his case.

If any allegations are flase then you file written complaint before the police and his senior officers and let the police collect the evidences and submit the report in court then fight on it.

What ever you say do that in writing to police and senior police officers, never say anything in words and also never agree anything on words....only in writing things shall be done.