Concerning higher Settler costs -- It appears to me that the cost of Settlers depends on the Era. In most games, there comes a point when I can build one in about 4 turns and at that point I crank out as many as I think I will need in the long run.
I've also noticed that there comes a time when I can pump out Settlers ridiculously quickly, but I always assumed it was due to a more developed capital city along with the +90% production bonus in said city.
I'll keep an eye out for that.