Tell a million people to go to take a grand to Vegas and bet their 1,000$ on 7 on a roulette table. Then have them take their winnings and do the same thing until FU money. Some of them would become millionaires, but collectively they would all be worse off. (They started with 1 billion.)
The secret is to look for things where risk > reward. Unfortunately writing generic web apps is probably not it. But, the obvious cost is simply time which people tend to undervalue, so it's not obvious if the game is rigged or not.