VoodooEconomist: Harpies' feathers and mutagens are the best source of income as far as I'm concerned - they weigh little and are expensive for their weigth, and also easily found. Which is a design decision I like - the best way to earn money for Geralt is to do a witcher's job, not a brawler's, merchant's or that of a craftsman.
Still, there's an option to meet Pangratt and get him to Vergen, maybe you'll be able to wrestle him then.
I'm pretty sure 200 orens in about 30 seconds is more profitable than farming harpies for their feathers, or doing sidequests (which I obviously complete anyway, but need more money)
Well, I didn't want to waste time with this, but I tried it anyway. No matter what you choose to do with the troll, Pangratt will behave in the same way once you talk to him in Vergen.
So on Iorveth's path it's pretty much sure now that you can't make a quick buck from arm wrestling.
I guess that leaves me with harpy farming or arm wrestling for 75 orens. :-(
Thanks for the tips ;-)
edit: oh yeah, I suppose the farming location is the old quarry? Because if it is, you gotta do it before destroying the harpy nests in the quest "The Harpy Contract" or otherwise you'll only meet 3 harpies. So farming is out too :-(