I see you're in California. There are a lot of charter schools around these parts that provide money to use for school-related expenses. This comes at the cost of having to abide by whatever oversite and testing requirements they have, but some are pretty laissez-faire.
Beyond that...
Between freecycle, giveaways within our homeschooling group (when curriculum junkies decide to clear out their stash), craigslist, and garage sales, I've had very good luck getting homeschool supplies either for free or very inexpensively. Thrift stores can also be good.
The internet has a ton of free resources, including lesson plans, unit studies, and printable worksheets on probably every subject imaginable.
Use the public library as much as possible. Not just for books... many library systems have videos, CDs, audiobooks, software, museum passes, and other resources available. Find out what yours has to offer that may not be common knowledge.
A big part of it is deciding on a budget and sticking to it, and making what you have (or can buy within your budget) work, rather than trying to find the perfect solution (AKA being a curriculum junkie

). Maybe a certain math program/boxed curriculum/whatever looks absolutely perfect... but if you can't afford it (either new or used) it's not perfect. Chances are there is a more affordable (or free) option that will work well enough, even if it isn't ideal.