I would say IMO a minimum of $2 per bag for a VERY short trip. $5 per bag is reasonable if they are taking you over the river and through the woods to the other side of the airport. I guess that would vary depending on the number of bags. If you have 8 big bags and you go a reasonable distance and the Sky Cap is friendly etc $15-20. OTOH I'd never tip a Sky Cap less than $10.