Wedding gifts

Hi all, My partner and I have been invited to the wedding of two people I work with. We have rarely socialised with them outside of work, but we have worked closely in the same team. For their wedding gifts they have asked for cash, what is a suitable amount to give? I really have no idea and would hate to come off cheap or offend them by not gifting enough. Does anyone have any guidance as to what a suitable amount would be? Thanks in advance.