It is hard to figure out. While some things like manufacturing, and tech can be converted to a formula, BCs to the AI fluctuate in value depending on how much they have at the time and other factors.
For instance, in the early betas you could buy ships from the AI for a song. Now it is all over the board, and it still makes little sense in many cases. In most cases it takes 2-3 middlin tech and 1000-1500 BCs to buy their only surveyor, (they will never build another). That's a lot, but a surveyor is worth at least 5-10 thousand BCs in found money over the life of a long game and that's 5-10 the AI won't find. Add that to the research and ship bonuses and the value of a surveyor goes through the roof, imo.
Throughout the betas, on normal difficulty, if I stayed in a game for 300 turns, I could usually just about buy everything I wanted. It has gotten harder since they changed the projects, but for the better players, (and hopefully me on normal)... I guess becoming a Merchant Empire requires that you transition from a Manufacturing Empire where you are constantly short of cash, to cash cow colonies that are matching the Mfg and tech that you have been producing all along.
Easy for some, hard for others. I can see that for some it is like baking a cake, just follow the recipe. For me it is more like spit-shining shoes. Sometimes I've got the mojo, sometimes not.