What is the fastest way to make money in this game?

I don't know if it's still relevant or you've already found it out by yourself (I've only just got the GOG version of it, so I didn't check the forum earlier), but I've found the fastest way is doing assignments.

I've personally found that as long as you have some means to profit on trips between systems (wether by assignments, escorts, trading or smuggling) while you're complelting story missions, hunting for artifacts and taking out pirate bases for secret service weapons gets you more than enough cash to buy whatever upgrades you could ever want or need.