For this achievement you need to hit a total amount (not cumulative) of 2 million credits.
One essential way to make money fast and relatively easy while exploring is to buy yourself, once you have about 450 Nanite Clusters, an S Grade Scanner Upgrade. This will drastically increase the amount of money you receive when discovering Flora and Fauna. I personally get a good 20000 credits per Flora scan and between 2600 and 146000 per Fauna scan. If you find a planet with lots of rare wildlife, you could hit a few million credits rather easily.
Update: You can install multiple Scanner Upgrades and they stack. You could, depending on your Multi-Tool, gain crazy amounts of Money from scans.
Another three ways to make a good amount of money fast:
• When you find an abandoned outpost that has Whispering Eggs near it, shoot an egg and collect the Larvae that it drops (it disappears rather quickly). Then simply use your jetpack to jump onto the structure before the swarm can get you. Wait a bit or shoot the swarm until is subsides. Rinse and repeat. Larvae Cores are worth (depending on the economy) about 95000 a piece and stack up to 5 in the regular inventory slots.
• Find a derelict freighter and open the cargo pods. While this is resource intensive, it always depends on the planet you are on and resources should be easy to find. You need to dig up some cargo pods as many are buried. Still, there is always one that is above ground. These contain resources and sometimes valuable resources that range up to over a million credits. While rare, you might as well try when you are there.
• Find alien ruins that holds keys. Where there are keys, there is a keyhole. On my first ruin, I found an artefact, some extinct plant sample, that I was paid nearly 700000 credits for. You will likely have to dig the keys and the container up.
Edit: Updated reward amounts.
Edit 06 August 18: Added note on stacking scanner upgrades.