You guys should tell her that this is our budget (whatever amount your family is looking to spend) so she should find something within that. If there’s a particular design she likes which is more expensive than the budget then tell her to get it copied from a good place so it doesn’t cost you guys an arm and a leg.
I have seen some girls being told by inlaws to stay within a budget while choosing clothes but if it costs slightly more, the girl or her parents pay the extra amount (if they can afford to) and not tell the inlaws 
This way the inlaws pay what they can and the girl is happy too.