It's not that there is a specific ammount of money you can make, but best you can do is:
Whenever choosing a cargo to deliver:
1)Choose the best combinations.
Don't just take one $1.5k cargo. You may be able to get like 2-3 smaller ones that will give you more money than that.
2) The locations.
The closer the detsination is, the faster you earn money.
3) Destinations in the same track.
So, if you've chosen a cargo to Bayside, and there's another well-paying cargo to Las Payasadas, choose it's on your way to Bayside.
That's it pretty much, the rest is up to you to drive fast - and also you need to be lucky to get some well-paying and lightweight cargos at the same time.