If it's being cumposed at a house, no more than a bill or a small kitchen appliance (foreman grill or magic bullet).
If its at a hall/restaurant... depending on how generous you are/money game on strongth... 100 dollars is minimum.
But then it also depends on your relationship with these people; good family/friends...
I try to drop like a good 100 or so.
Why don't you just seranade the couple with a vocal gem?
Trust your heart, braye.
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