A site I like called Park Whiz gives you the ability to have a reserved parking spot ahead of time. This isn't usually a budget option per se, but there is some value in the peace of mind knowing you have a spot with your name on it. Check out Park Whiz to see what is a available for the playoffs.
If you can't get a reserved parking spot, think about these other options.
If staying in San Francisco, you can take the Muni Metro. Lines N and T go directly to the ballpark and let you off on King St. Lines 10, 30, 45, & 47 also stop near the park (one block away from AT&T)
• If coming from the Peninsula or the South Bay area, you can pick up CALTRAIN throughout Santa Clara and San Mateo and take it within one block of the ballpark. In addition, you can ride BART (Bay Area Rapid Transit) from Milbrae, SFO (Airport), South San Francisco, San Bruno, Colma, or Daly City to downtown San Fran and transfer to the MUNI METRO N or T @ Civic Center, Powell, Montgomery, or Embarcadero stations
•For the regular season you were able to ride various ferries to the park from other parts of the bay area as well. From the East Bay (Alameda and Contra Costa), you can take the Alameda/Oakland Ferry.to the park. In addition, from the North Bay, the Lakespur Ferry will bring you to the park from the Marin and Sonoma areas. For the playoffs we don't have the details on reservations or schedules, but it may be worth calling about on the sites linked above to see if they plan to offer service during the playoffs.
Again - make sure you have a plan and arrive plenty early!