-While starting the game and still in Liberty, you can find a good profitable trade route between Planet Manhattan and Planet Pittsburgh. Fill up on Pharmaceuticals at Manhattan, then sell them at Pittsburgh. After that, fill up on Boron at Pittsburgh and sell them at Manhattan. Keep doing this to easily get enough money to upgrade your ship to a Rhyno. You can earn nearly 5,000 credits for the whole round trip.
-If you are patient, try this trade route. Purchase Gate/Lane parts at Trenton Outpost in New York. Take this to Holman Outpost in the Tau 31 system and sell it, then buy Niobium at Holman Outpost and take this to Planet Manhattan (which is next to Trenton Outpost). Sell it, then go back to Trenton Outpost and repeat the process. A HumpBack freighter can earning 347,500 credits in one round trip. If you prefer to keep a Heavy Fighter, you can still earn 97,300 credits in one round trip.
-The following is a cheap and easy trade route. It only applies to Freelancer v1.0 (v1.4 Cap ships mod is acceptable). Make a new ship/character on a multi-player server (or if you want to test it, try a single player game if you have completed Story mode). This isn't the fastest method, but it does work for when you start out with only 5,000 credits (if the server does not edit that). Fly to the Leeds system and go to Stokes Mining Station. Buy Ship Hull Panels for 80 credits each (1,600 credits for 20 units with the Starflier, your starting ship). Then, fly back to the New York system and land in the Baltimore Shipyard. They will sell for 560 credits per unit (11,200 credits for 20 units). You can now purchase at least a Rhino and sell even more at once. Repeat this as many times as desired.