You are not under oath to bear any extra gifts if you attend a wedding with a date or group of friends.
But if you have a close relationship with the couple, you are morally bound to come with a gift double the cost or a bit over one adult head.
This is to show support to the couple and pay for the extra plate of food.
As such, really close friends with a plus one should spend about $180 – $250. For others $80 – $160 is fine.