I was able to change my tariff rate in my current game by changing my save file. There are a lot of rates to change, but I used notepad +++ and I also changed in in the overall galaxy, so if I generate a new game, I'll be set. A dynamic rate change between 5-30% depending on global condition would be nice and would encourage dynamic trading, rather than just boring single route...dunno why this idea wasn't implement, unless they wanted battles, battles, battles. I set all of my tariff rates to 10%, btw. 30% all the time for both buying and selling is absolutely ridiculous, imho.