http://www.youtube.com/watch?v=eept-AGdhNA
THat is from MLB 12, but it works the same.
You gain budget by playing basically. You can also get budget by doing collections and buying/selling from the market place.
The better and more you win, the more budget you earn. You have a thing called fan support that gives you so much budget per inning based on well your players do individually. You also get more budget by finishing games and winning.