Best bribe amount for switching aisle to aisle seat in F?

What is the best Appropriate bribe for switching aisle to aisle seat in F? 4 hour flight on United, Sfo to ord.
we all hate seat poachers. I’m going to buy last-minute seats and they’ll be separate for my minor child and me. What’s a fair amount to offer when asking the other person? I don’t want to be in Y or Y+

i am thinking $20 for 2 future cocktails