I think what we call common knowledge may be wrong at this.
We usually pay 10-20% of what we order as tip, but let's say this.
If a family with four people order 4 $10 menu with 2 or 3 dishes each, a server would get $4-8 as tip after delivering around 10 dishes.
If a person orders a $80 steak, a server would get $8-16 as tip after delivering around just 1 or 2 dishes.
Who should get paid more?
Why do servers or customers think customer should pay tip not by the service but by the amount of money spent?
I don't want to say how much tip would be proper for a dish, but I hope we could think about it.
If we think paying tip by service, we could pay it as much as the service is provided.
If we think paying tip by spending, we may not need this discussion.