The short version
TwisterX Farming Guide: How to Earn Money: Money in TwisterX comes from storms: radar trucks scan them for cash (the DOW 6 pays more per scan than the DOW 3, on the community wiki's numbers), and $1,000 probes record a tornado's maximum wind speed when it passes over. The ladder is Twistex Turtle at $1,000, then DOW 3 at a reported $200K, then the DOW 6 at $500K — and thanks to the one-time purchase rule, crossing a threshold unlocks the next truck without emptying your balance.