The last time I was at a lounge, between the two of us we got one soda and the rest was water. After 3+ hours my bill was $5. A waiter or waitress is tiped for their service, a drink is all the same to carry regardless of whats in the cup. We use percentages as a metric but it shouldn’t be a hard and fsst rule. I tipped the waitstaff for their wonderful attentiveness with $15 or $20. It’s just rude otherwise and very possibly can be a chillul hashem.
So if the two of you had also ordered one $2000 drink, you would have tipped $15 or $20?
Obviously not. Because percentage is more than a metric.