When people say "labor of love" what they really mean is "if you can spend the money and not miss it if whatever you get goes bust, then go for it."
If you can drop $500 and, if the bass sucks, or you guys wreck it somehow, and you've got parts and tools already in order to try to finish it up, then yeah, knock yourself out.
However, if you'd have to buy tools, varnish, clamps, glue, chisels, and so on, and this is a one-off thing, then the cost is not $500 -- it's bass + finishing tools.
For comparison, my wife recently bought a violin. She was shopping around locally and on Craigslist, and if she didn't want something that sounded horrible, she was looking at around $500 -- for a starter model. So she figured she'd take a chance, and bought a new violin from Yita music. Yita music actually has come up here, as well, with some people trying out their bows and generally saying that they're pretty good, for the price. It's the same with their violins -- sometimes you can get a dud, but you usually end up with an instrument that's much nicer than a comparable one you'd buy locally. Just how it is.
So she instead spent $200, and spent $110 getting it setup -- fingerboard planed, post set, and so on. She now has a pretty nice violin, still for significantly less than if she had bought a relatively crappy model.
The point is that she did the calculations and said to herself (and me), "If I get an OK violin, as long as I spend under $200 in setup to fix any problems that crop up, I got a good deal." And it's worked out -- she got a good deal.
But she also approached it with the point of view that if she got a dud, $200 would be an OK amount to get a starter instrument, and then move up to a nicer instrument after 6/12 months once she knew what to listen for (and once she was good enough that she felt spending the money was justified).
So at what point is this bass project no longer a "good deal" for you? If the wood itself is comparable to a $600 bass-shaped object, then it's probably not a good deal -- your time is worth something, after all. If it seems like a decent deal, just "in the white," how much do you estimate you'll spend on the rest of the parts? If that's still in "good deal" territory, then ask yourself "If this is a bust, is that OK?"
One of the reasons most musicians will suggest you listen to an instrument before you buy is that there's risk involved -- that you won't like the sound. That risk goes up on cheap instruments. (Of course, the risk on the other end is that it's a ton of money!)