Free Trade and the Price of Mills

Since this heavily relates to economics, I decided to post it here, where I expect a lot of people to know about it.

Before the free-trade update, I remember than mills were 5usd~`0usd ea. But now, they cost less than 2usd. Probably because of the update.

So I was wondering, if free trade returns, will the prices of mills increase or decrease?

 

I think the prices will decrease at first, but after some time start increasing and stay stable then.