If you have a second colony set up (Mars in this case) then the civillians will get into the act. This means it is worth establishing a seed colony (10-30K people) which they will then take over growing for you. So you get Mars settled for around the cost of 50-100 infrastructure (or less) that you buy.
The shipping lines have access to civillian built infrastructure so that part of the expense is for free. Given it takes ages to do any sort of terraforming there is no reason to wait to establish a colony.
You need a full 5 cargo holds on one ship to move mines, factories and other such things, they don't split up.
Once the colony is established the civillians will move people and infrastructure to it, this generates money for you, and for them. Once the colony gets to a few million people it develops its own economy (produces trade goods) which you then tax when they are shipped. So to me, anyway, establishing a colony at cost of 2-3 is not a bad idea. After that though it gets dicy as the civillian lines don't think before they drop so there will be lots of wasted colonists on high colony cost world.
This applies interstellar wise as well so long as a gate exists for the civillians to use.