Easy to understand summary from a BFF (Big F* Fanboy)
Seriously it must be priceless for Elon to have people like this on board with his dream. Which fingers crossed becomes a reality.
I can't imagine there would actually be much trade between the planets. I'd think the two would develop separately. The monetary factor which allows it to get started is that people simply want to go, but it's hard to imagine it costing less to purchase something from Mars in the next century.
If sending stuff around, even if it takes a long time, is cheap enough, they could say trade seeds and minerals between each planet and have a constant flow of small cheap cargos each way. Think about the old trading routes in Age of Empires.