TerraTech

The most valuable biome in the game is the Pillars biome. They're a bit rare but if you manage to find one, you'll be able to make a ton of money no matter the size you get. This biome is covered with these pointy rocks that you can shoot and mine Ignite, Erudite, and Celestite from. These are the 3 most valuable ores in the game. These ores can show up in other biomes but Pillars has the most abundant quantity.

EACH SINGLE ore, when refined, gives 225 bucks for Ignite, 233 for Erudite, and 240 for Celestite. That sounds like a little bit compared to a million, but clearing out one instance of this biome will rack up thousands of bucks very quickly. Then all you have to do afterwards is go hunting for another one, which you can do the fastest by building a small airplane and travelling by air.

Here's a couple of images of what a Pillars biome looks like.