Likely the best anyone can do in the game of Yugioh is hit the "break even" point...that meaning, you win enough boxes of cards and a few other things that you don't have to buy cards anymore and have enough boxes of cards and singles as well as perhaps a laptop or IPOD to sell to break even on your travel expenses.....that's if you're one of the top 4 players in the nation on a consistant basis.
It's entirely possible and more likely that if you play on a state / regional level and very good you could hit this break even point. Traveling outside of the state for tournaments gets expensive quickly.
As previously mentioned, VS, MTG, and Dreamblade offer the big money prizes and are likely the only means of making a real profit off of playing the game....however, if you're a sharp business person you're more likely to make money selling the cards than playing them....lots of opp. and pretty easy to start too selling cards.