Bio-weapons are the most profitable junk in the game, and one of the really nice complexities that make Escape Velocity so insteresting. I worked it out a heuristic trade route by writing all the planets on the geodesic between Codehaven and Lesten (I could have done it for Moash, but that's more complicated) and finding all cargo that could give profit on the way back.
It works better if you have escorts that can carry cargo. I recommend first starting with Terrapins, then upgrading to Pegasuses and Leviathans. Alternately, you could go for Cambrians. The cargo is only 3500 tons versus 4000 tons but the cost is nearly one fifth; go thought the wormhole from Sol to Ver'ashan if you decide to take that route. Of course, you could capture ships, but I haven't tried it.
Buy bio-weapons at Codec. Hold down option-alt -- you don't want to be there all day.
I like to accept Bounty Hunter missions to spice it up; good for the legal status too.
Sell bio-weapons at Menin. Menin starts off the game mis-trusting you, so you may need to bribe someone to let you land there. Buy equipment from from Menin.
Sell equipment at Georgia in the Nerse Primus system. Buy industial; it's more then likely "lower."
Sell it at Earth and buy medical supplies.
Sell it at Sirius II on the way back to getting more bio-weapons.
Once you get going, you can make about thirteen million credits every cycle and the people will love you [supplying bio-weapons.]
If you have access to the Hypergate system, it becomes much more difficult to say if the trade route is optimal. I like to sell bio-weapons at Moash, and buy medical supplies at Pyrogenisis I in the Nerse Secundus system, take the Hypergate to Kerella and sell them at Sirius II. I calculate 23.60M and 7 days with the Medical Supplies verses 16.94M and 5 days with the Bio-Weapons alone. It becomes hard to even apply a heuristic to say that this is one of the optimal trade routes in EV Nova, however.
I did it with captured escorts; 35.49M going the way I desribed and 23.50M going to Moash. The Leviathans cost a lot. Now that I have time, BioWeapons Codec-Menin, Equipment Menin-Europa, Med Supplies Earth-Sirius II; this gets me 36.94M in two days less. Serious contender for Moash. I like.