Tax System blues

Ok, just when I thought I had the system figured out I've been thrown a loop. Giran currently has a tax rate 5% higher (15%) than Dion, yet, consumables and D items sell at a higher price in Dion than Giran.

I was under the impression that Dion and Giran both were taxed by Aden, but is Dion taxed by Giran as well? I can't seem to find the map I once saw regarding what castles got pieces of other cities' pie.

Each city also has a base tax rate, giran's is the lowest so even at 15% taxes it's cheaper than many others.