I always view a port as money well spent. If A to B takes me 30-45 minutes on foot I make sure I make it worth at least close to 30-45 minutes of my time. There aren't many A to B routes that take this long unless crossing oot.
Amount I donate fluctuates depending on how much cash I have on hand and in the bank. If I'm sitting at more than 3k (I'm poor lol) I think the most I donated for a random port was 150p, average 50+ still if I'm stretched for cash.
If I'm in a weird place requesting a pickup and port to a random place I've donated as much as 200-300.
But I'm poor
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For ejacs I find it more productive to hammer to OT and run to KC and find a Druid etc who's in the process of doing ejacs. If I'm not having them escort me from wherever to DL and then to KC for ejac, I don't mind just paying them 50p for the evac alone.
Lower levels get dicey. If you're not a bankrolled twink, getting even 5-10p for a cheap port can strain your bank. These days I try to make sure my alts have at least enough cash for a respectable donation to a location or two I know I will be visiting in the early levels and then minimize the traveling I do.