As a general guideline for *myself,* I try to give enough to at least cover the cost of what I’m going to eat and drink at the event. Just think about what you would spend at a restaurant for a similar type of meal and atmosphere, and it makes it easier to come up with an amount. But no one expects you to exceed your budget. A struggling student should feel comfortable giving a small €10 gift, but a middle aged person of comfortable means might give €50-€100, and a close family member might spend even more if they want. Go with your instinct. I will say I have never regretted erring on the side of generosity.