My favorite system uses chaos theory. All you do is wait for a hurricane. Then you do the math, and it can predict when a butterfly will flap it's wings half way around the world. It takes about fifteen years for the cause to produce the effect, which is good, because with the slow computers we have today, that's about how long it takes to do the math. And about that time, another hurricane occurs, so you are kept quite busy. You can use it for trading, but most people who truly understand chaos lose interest in mere dollars.
Ask if the developer trades his own system and ask for account statements. I guarantee you that you won't get any replies.
I paid option40 people $300 for a system with 2 numbers but it got me nowhere (trading option onOEX).