I've been stiffed by several HG's in a row but still kill them. If I get stiffed a couple times in a row by lvl 50+ non CR ports hot off hammer hill, I go back to porting HG's
[You must be logged in to view images. Log in or Register.]
There's a lot of factors on determining what would be an 'appropriate' tip for a port, and it isn't really something you can put a set rate on. Basically (like Cecily is elluding to), if folks are tipping well enough, porters will be more disposed to sitting at rings waiting to port you, rather than camping some xp/pp somewhere else.
It's not so much the cost of mana for the port, but more the time sink in being there available to port you when you're in a hurry running down to SF ramp...