Not a "stupid question" at all! This is an important question, actually, since many people who have never homeschooled wonder all the time how they are going to manage. Believe me, I've definitely been there!
If your family is currently relying on a dual income, some creative changes are going to need to be made. One option is to have an opposite shift from your partner, if you are not single, of course. Another is to work from home; not all at-home jobs are scams, and starting your own business *is* an option, though may not be lucrative in the immediate future. A third is to work part-time and figure out a way to cut back on expenses. And of course, the fourth is to go down to a single income and *really* reevaluate your budget.
As far as the day to day costs go, homeschooling can cost as little or as much as you want! Seriously! You have Internet access, so you already have the world at your fingertips! Plus, libraries are also free, as are playgrounds, and zoos and museums are always offering free days if you look them up.
Since our state gives us a tax break on school expenses, we actually keep a record for our CPA. Our 2008 filing reflected only about $400 spent over an entire year, which included a lot of trips to the bookstore (with a teacher's discount), a Casio keyboard, parking fees, a couple computer programs and the necessities any student can expect, like paper and ink
In contrast, I know for a fact I spent a LOT more back when our son was in public school, probably twice as much. There was always a fundraiser for one thing or another (which I never understood), school supplies that were never even used, field trip expenses, lunches, bus fare, not to mention that mad dash to buy up back to school wardrobes. Our guy is still stylin' some sweet kicks (lol), but because we doesn't have to worry about keeping up with the Joneses on a certain day, I can get them for him when they're 50% off or better.
I also have noticed over the years he doesn't burn through his allowance money, but rather saves it up for things he really wants. When he was in school, he would spend his $20/week as soon as he got it, and it was usually on junk food. Now he saves up for several weeks to build up on his turbo-charged computer, games, fitness equipment, or yes, the occasional Friday night pizza
Hope that helps!!