Are you going alone or with someone, and is it more of a fancy wedding or something more laid-back?
Because honestly, the budget really depends on the context. With friends, people generally give between 50 and 150 euros, but it varies a lot. If it's a wedding in a venue with a meal and all that, you can lean more towards 80-120 euros. If it's something simpler, 50-70 works just fine. The idea is that the gift covers roughly the cost of the meal and drinks they're paying for you.
The thing is, don't compare yourself to the others in the group. Yeah, there are several of you friends, but everyone has their own financial situation. Nobody's going to check how much you gave, and your friend is mainly just going to be happy you're there. Honestly, if you're a whole group going, you can even coordinate quietly with one or two buddies to give something a bit nicer together - like an actual useful gift instead of just cash. It makes the whole thing feel less cold and it's usually more appreciated.