You can make 50-60 million in an hour or two without leaving the Earth sector.
Buy FC Charger Units at Galcom and sell them at Genesis (the moon's station). Buy Commlink units at Genesis, and take back to Galcom.
When you get your first couple of million, start buying Cloaking systems at Galcom for Genesis.
Eventually you can start hauling Cloning Units to Genesis, these net you ~900,000 credits per unit for a simple hyperjump.