Wedding gift

My husband is a groomsman in his step brothers wedding. Long story short they kind of hate each other but he agreed to be a groomsman to keep the peace in their family. We didn’t realize how much we’d have to spend on attire/ travel/ accommodations, etc when he agreed but it’s too late to back out now and we’re already stretched thin. We have a 2 month old little girl and I am on mat leave and we are barely getting enough income to pay our bills. So my question is how much money is appropriate to put in a wedding card? The couple is kind of “family”, so are we expected to give more in this situation? If so how much? We really can’t afford much but I don’t want to seem cheap either 😞