The value of anything is going to end up being what both party's of a trade agree upon, but by choosing 1:1 same-tier everything is super fast and easy.
Sure, you can do dozens of calculations and watch the trader and add up the costs in each chapter and and and, but most players aren't here to play "Market sim 2000" So we do what's easy: a little bit of "equal outcome for equal effort" and no one ever feels cheated that way.
If 2 players perform the exact same actions and have the exact same outcome it's really hard to call it unfair.
If no one feels cheated at the end of a transaction, that's close enough to "fair" for me.
I have yet to see an alternative methodology, while everyone can see at a glance if a trade is 2-star and same-tier so it's super easy to go by.
Feel free to recommend a consistent, easier, more reliable measurement of fairness, and I'll think about switching.
Obviously you believe the game should be easy. Or at least easier when it comes to trading. Just look at the stars and let the game (in essence) make the decision for you -- 2 stars and done. But of course which building to put in -- which is more productive per square than another -- you have no problem encouraging people to do that math. And which combination of AW's, event buildings, regular buildings, roads and so on and so on, should you use? No problem, just do the math. The thing is, trading is NOT about math, but about a sense of value and that is a personal decision. Like I keep saying, there are intangibles -- like how important is it to you at this time to finish this quest in this event? Or how important is it to you to cater this spire chest at this time? The list of intangibles is what makes any cut and dried formulation for measuring trade go out the window as soon as it's presented. In fact, if you've ever done any real world market trading you'll find very little complex calculations done. Most traders develop expertise -- intuitive understanding -- of what's going on in their market segment and then look for changes in the details -- like the management team in place for company X is moving to company Z, or that this or that ship just went aground, or company W is opening a new factory in Tibet and will need a lot of tungsten. Each of these, they know from experience will affect the stock price of this or that company. They predict based upon experience, not complex calculations (though a few do try some of that as well -- and don't do any better than the intuitive folks, and sometimes worse).
So, no need to play "market sim 2000" but instead be aware that a free market is more predictable than you think just as knowing you should only buy (at this time) scrolls for less than silk or crystal becomes intuitive after you've played a while.
And you are right that IF the devs had insisted and made it impossible to vary from the 1:1 ratio trades everything would be considered fair. But then nobody would be buying scrolls and us boosted scroll producers would have to build non-boosted silk and crystal and I'm willing to bet, have left the game a long, long time ago out of real frustration. Since the devs didn't insist on the 1:1 ratio and let us vary from it, perhaps they too, realized that there are market forces at work in spite of their desire that the markets be "fair" by their own standards. Everyone makes mistakes, even the devs.