I just bought the game and read the rule book. There is one question:
As far as I understand it, the trade count cannot drop below the coin count by researching. So let's say I have 6 or more coins and the exact same number of trade and want to pick up a level 1 tech. Can I basically take it for free?
I tried to look it up and it does not seem to violate any rule. There is just an example where coins are used to save surplus trade so that the full price for the tech has been paid.