Well, let's talk about the rule in question, rather than anything else.
I think its fair to say that the system is designed to encourage trade, and to give traders to reserch that advance a competitive edge. So, if you've gotten to the point where you get 10% back (much costlier reserch, but easier math) you can sell at cost and cover your over head, or sell at what your buyer thinks is your cost (including terriffs) and make a proffit, or what ever.
The problem is, it is based on enhancing revenue. In effect, it makes 'free' money appear. Where does the extra 5-10% come from? Naturaly, its based on exchange rates and maket demand and other things, IC, but game mechanicaly its a bit funny. And anything that's a bit funny attracts people that, let's be honest, are like me (again, read into that what you will, and weigh that against the past decade I've been in this game).
Would a simpler mechanism not be: reduce costs? That is to say, as you reserch, your terriffs go down (streamlined bourder crossings, more efficant systems, knowing who to bribe and how much, etc) or transport costs go down (more efficant transport hubs, better invintory managment, less waste, etc). So at even at the maximum point, you don't make any 'free' bonus money by trading, you just don't have to spend any of your own money to do it either.
If we apply a bonus to transport costs, this actualy can have a domestic effect that goes back to that factory thing we discussed earlier, too, making factory spacing an even more nuanced proposition. Though that is not essental for the fix to function, and could be left out if it was felt to be imballancing.
But, since you can't increese proffit, just reduce cost (and never below 0) I would offer that such a system achives the same thing (incentives to trade, and advantages in trade to the possessers) with fewer oppertunities for abuse; there is now no possible way that nations can colude to make magic free money.
