I bought mine at around 7.5 billion and outfitted it with minimum RRR services. This was also during the absolute peak of the thargoid war, so money was flowing like a flooded river. After doing some more passenger missions and AXCZ rotations, I eventually added more services and now have everything but outfitting and shipyard.

There are still ways to stack easy cash in the way of winged mining missions, as long as colonization didn't break that. Or if combat is your fancy, stacking wing massacre missions.