If you have the machinery to mill rough boards (i.e. table saw, planer, jointer), you can get wood very inexpensively from a saw mill. Not sure where you're located, but chances are good there's a saw mill not too far from you. I'm in the middle-of-nowhere-Iowa and I have 5 or 6 decent sized saw mills within an hour's drive. With as inexpensive as the portable mills are these days, there are more and more guys running small operations, and in my experience their prices are even lower.
My first builds I used poplar and maple from a local saw mill. They had curly maple for about $4/bdft, and I think the poplar was under $2/bdft. Considering you don't need much wood to make a bass, you can come out fairly inexpensively. I think I paid less than $40 for the wood to make two basses (body, top, neck and fingerboard). Considering how much things like hardware and electronics (heck, even strings) cost, the wood might end up being the least expensive part of the build.
It's when you start looking at the more "exotic" woods that things get more expensive. Even so, the local "exotic wood" lumber guy I use will cut me off a 2' piece from the end of a board, so I don't have to buy an 10' long piece of zebrawood at $20/bdft.