I'm not trying to talk you out of unschooling :LOL I think it's cool myself
But if you want to go the structured route, I think you can still do it on a budget. You just have to plan more.
First, you can buy used curriculums online. I know that vegsource.com has a homeschool swap board. There are probably others. eBay maybe?
Second, you can use the library. From what I've heard, there are curriculums out there that don't give you all the books - they just tell you which ones to go get. I think Well Trained Mind
is like this? Also, What Your (Kindergartener, First Grader, etc) Needs to Know
is sort of like that. It covers a lot and then gives suggestions on further reading. So if you wanted to do a structured plan like that, all you have to do is buy one book and then go to your library for everything else.
If you're just looking at the cost of text books then yeah, unschooling is cheaper. Uh, unless your unschooled child requests textbooks of course :LOL But even among structured homeschoolers there are a lot who prefer not to use textbooks. There's a lot of stuff you can print out for free on the web (especially the lower grades) and websites that can help you learn almost anything out there.
Good luck with whatever route you go