I was doing some calculations and came up with this
with a type E trade (Aluminum, coal, iron, lead, lumber, marble, oil, rubber, water, wheat, gold,*)you get 42% less inf cost,
with interstate you get 8% less,
with 5 factories you get 40% less,
monarchy 5% less
totaling 95%
with the most recent addition to wonders the Scientific Development Center lowers it another 10%
105%??????
* last one is economic and doesn't matter (normally gems, fish, or wine)